make: switch the PYTHON default to `py.exe -3` on Windows

Python3 _is_ named `python.exe` on Windows, but that isn't necessarily on PATH
when installing from python.org.  I do happen to have a python.exe on PATH in
`$LOCALAPPDATA/Microsoft/WindowsApps`, but it appears to be 0 bytes (likely
because of permission issues), and doesn't run:

    $ python -V
     - Cannot open

Pulkit hit the same error as I did though, so it isn't just my system:

    $ make -C . local
    make: Entering directory `/home/Dell/repos/hg-committed`
    python setup.py  \
              build_py -c -d . \
              build_ext  -i \
              build_hgexe  -i \
              build_mo
     - Cannot openmake: *** [local] Error 1

The `py.exe` dispatcher lives in the Windows directory (so it is on PATH), looks
up the python.org installation, and invokes that interpreter directly.  I get a
warning with py39, but if it's our issue, it was an existing one:

    $ make -C .. local
    make: Entering directory `/c/Users/Matt/hg'
    py -3 setup.py  \
              build_py -c -d . \
              build_ext  -i \
              build_hgexe  -i \
              build_mo
    C:\Users\Matt\AppData\Local\Programs\Python\Python39\lib\site-packages\setuptools\distutils_patch.py:25:
        UserWarning: Distutils was imported before Setuptools. This usage is discouraged and may
        exhibit undesirable behaviors or errors. Please use Setuptools' objects directly or at least
        import Setuptools first.
      warnings.warn(

The end result is a py3 based hg.exe that annoyingly won't run because it can't
find python39.dll.  It will run tests (the ones without the `python3` shbang
line anyway), because the test runner adjusts PATH to include the python running
it.

Differential Revision: https://phab.mercurial-scm.org/D9361

Makefile

@@ -1,268 +1,268 b''
 # If you want to change PREFIX, do not just edit it below. The changed
 # value wont get passed on to recursive make calls. You should instead
 # override the variable on the command like:
 #
 # % make PREFIX=/opt/ install
 
 export PREFIX=/usr/local
 
 # Default to Python 3.
 #
-# Windows ships Python 3 as `python.exe`.
+# Windows ships Python 3 as `python.exe`, which may not be on PATH.  py.exe is.
 ifeq ($(OS),Windows_NT)
-PYTHON?=python
+PYTHON?=py -3
 else
 PYTHON?=python3
 endif
 
 PYOXIDIZER?=pyoxidizer
 
 $(eval HGROOT := $(shell pwd))
 HGPYTHONS ?= $(HGROOT)/build/pythons
 PURE=
 PYFILESCMD=find mercurial hgext doc -name '*.py'
 PYFILES:=$(shell $(PYFILESCMD))
 DOCFILES=mercurial/helptext/*.txt
 export LANGUAGE=C
 export LC_ALL=C
 TESTFLAGS ?= $(shell echo $$HGTESTFLAGS)
 OSXVERSIONFLAGS ?= $(shell echo $$OSXVERSIONFLAGS)
 CARGO = cargo
 
 # Set this to e.g. "mingw32" to use a non-default compiler.
 COMPILER=
 
 COMPILERFLAG_tmp_ =
 COMPILERFLAG_tmp_${COMPILER} ?= -c $(COMPILER)
 COMPILERFLAG=${COMPILERFLAG_tmp_${COMPILER}}
 
 help:
 	@echo 'Commonly used make targets:'
 	@echo '  all          - build program and documentation'
 	@echo '  install      - install program and man pages to $$PREFIX ($(PREFIX))'
 	@echo '  install-home - install with setup.py install --home=$$HOME ($(HOME))'
 	@echo '  local        - build for inplace usage'
 	@echo '  tests        - run all tests in the automatic test suite'
 	@echo '  test-foo     - run only specified tests (e.g. test-merge1.t)'
 	@echo '  dist         - run all tests and create a source tarball in dist/'
 	@echo '  clean        - remove files created by other targets'
 	@echo '                 (except installed files or dist source tarball)'
 	@echo '  update-pot   - update i18n/hg.pot'
 	@echo
 	@echo 'Example for a system-wide installation under /usr/local:'
 	@echo '  make all && su -c "make install" && hg version'
 	@echo
 	@echo 'Example for a local installation (usable in this directory):'
 	@echo '  make local && ./hg version'
 
 all: build doc
 
 local:
 	$(PYTHON) setup.py $(PURE) \
 	  build_py -c -d . \
 	  build_ext $(COMPILERFLAG) -i \
 	  build_hgexe $(COMPILERFLAG) -i \
 	  build_mo
 	env HGRCPATH= $(PYTHON) hg version
 
 build:
 	$(PYTHON) setup.py $(PURE) build $(COMPILERFLAG)
 
 wheel:
 	FORCE_SETUPTOOLS=1 $(PYTHON) setup.py $(PURE) bdist_wheel $(COMPILERFLAG)
 
 doc:
 	$(MAKE) -C doc
 
 cleanbutpackages:
 	rm -f hg.exe
 	-$(PYTHON) setup.py clean --all # ignore errors from this command
 	find contrib doc hgext hgext3rd i18n mercurial tests hgdemandimport \
 		\( -name '*.py[cdo]' -o -name '*.so' \) -exec rm -f '{}' ';'
 	rm -f MANIFEST MANIFEST.in hgext/__index__.py tests/*.err
 	rm -f mercurial/__modulepolicy__.py
 	if test -d .hg; then rm -f mercurial/__version__.py; fi
 	rm -rf build mercurial/locale
 	$(MAKE) -C doc clean
 	$(MAKE) -C contrib/chg distclean
 	rm -rf rust/target
 	rm -f mercurial/rustext.so
 
 clean: cleanbutpackages
 	rm -rf packages
 
 install: install-bin install-doc
 
 install-bin: build
 	$(PYTHON) setup.py $(PURE) install --root="$(DESTDIR)/" --prefix="$(PREFIX)" --force
 
 install-doc: doc
 	cd doc && $(MAKE) $(MFLAGS) install
 
 install-home: install-home-bin install-home-doc
 
 install-home-bin: build
 	$(PYTHON) setup.py $(PURE) install --home="$(HOME)" --prefix="" --force
 
 install-home-doc: doc
 	cd doc && $(MAKE) $(MFLAGS) PREFIX="$(HOME)" install
 
 MANIFEST-doc:
 	$(MAKE) -C doc MANIFEST
 
 MANIFEST.in: MANIFEST-doc
 	hg manifest | sed -e 's/^/include /' > MANIFEST.in
 	echo include mercurial/__version__.py >> MANIFEST.in
 	sed -e 's/^/include /' < doc/MANIFEST >> MANIFEST.in
 
 dist:	tests dist-notests
 
 dist-notests:	doc MANIFEST.in
 	TAR_OPTIONS="--owner=root --group=root --mode=u+w,go-w,a+rX-s" $(PYTHON) setup.py -q sdist
 
 check: tests
 
 tests:
         # Run Rust tests if cargo is installed
 	if command -v $(CARGO) >/dev/null 2>&1; then \
 		$(MAKE) rust-tests; \
 	fi
 	cd tests && $(PYTHON) run-tests.py $(TESTFLAGS)
 
 test-%:
 	cd tests && $(PYTHON) run-tests.py $(TESTFLAGS) $@
 
 testpy-%:
 	@echo Looking for Python $* in $(HGPYTHONS)
 	[ -e $(HGPYTHONS)/$*/bin/python ] || ( \
 	cd $$(mktemp --directory --tmpdir) && \
         $(MAKE) -f $(HGROOT)/contrib/Makefile.python PYTHONVER=$* PREFIX=$(HGPYTHONS)/$* python )
 	cd tests && $(HGPYTHONS)/$*/bin/python run-tests.py $(TESTFLAGS)
 
 rust-tests: py_feature = $(shell $(PYTHON) -c \
  'import sys; print(["python27-bin", "python3-bin"][sys.version_info[0] >= 3])')
 rust-tests:
 	cd $(HGROOT)/rust/hg-cpython \
 		&& $(CARGO) test --quiet --all \
 			--no-default-features --features "$(py_feature) $(HG_RUST_FEATURES)"
 
 check-code:
 	hg manifest | xargs python contrib/check-code.py
 
 format-c:
 	clang-format --style file -i \
 	  `hg files 'set:(**.c or **.cc or **.h) and not "listfile:contrib/clang-format-ignorelist"'`
 
 update-pot: i18n/hg.pot
 
 i18n/hg.pot: $(PYFILES) $(DOCFILES) i18n/posplit i18n/hggettext
 	$(PYTHON) i18n/hggettext mercurial/commands.py \
 	  hgext/*.py hgext/*/__init__.py \
 	  mercurial/fileset.py mercurial/revset.py \
 	  mercurial/templatefilters.py \
 	  mercurial/templatefuncs.py \
 	  mercurial/templatekw.py \
 	  mercurial/filemerge.py \
 	  mercurial/hgweb/webcommands.py \
 	  mercurial/util.py \
 	  $(DOCFILES) > i18n/hg.pot.tmp
         # All strings marked for translation in Mercurial contain
         # ASCII characters only. But some files contain string
         # literals like this '\037\213'. xgettext thinks it has to
         # parse them even though they are not marked for translation.
         # Extracting with an explicit encoding of ISO-8859-1 will make
         # xgettext "parse" and ignore them.
 	$(PYFILESCMD) | xargs \
 	  xgettext --package-name "Mercurial" \
 	  --msgid-bugs-address "<[e-mail]>" \
 	  --copyright-holder "Matt Mackall <[e-mail]> and others" \
 	  --from-code ISO-8859-1 --join --sort-by-file --add-comments=i18n: \
 	  -d hg -p i18n -o hg.pot.tmp
 	$(PYTHON) i18n/posplit i18n/hg.pot.tmp
         # The target file is not created before the last step. So it never is in
         # an intermediate state.
 	mv -f i18n/hg.pot.tmp i18n/hg.pot
 
 %.po: i18n/hg.pot
         # work on a temporary copy for never having a half completed target
 	cp $@ $@.tmp
 	msgmerge --no-location --update $@.tmp $^
 	mv -f $@.tmp $@
 
 # Packaging targets
 
 packaging_targets := \
   centos7 \
   centos8 \
   deb \
   docker-centos7 \
   docker-centos8 \
   docker-debian-bullseye \
   docker-debian-buster \
   docker-debian-stretch \
   docker-fedora \
   docker-ubuntu-trusty \
   docker-ubuntu-trusty-ppa \
   docker-ubuntu-xenial \
   docker-ubuntu-xenial-ppa \
   docker-ubuntu-artful \
   docker-ubuntu-artful-ppa \
   docker-ubuntu-bionic \
   docker-ubuntu-bionic-ppa \
   fedora \
   linux-wheels \
   linux-wheels-x86_64 \
   linux-wheels-i686 \
   ppa
 
 # Forward packaging targets for convenience.
 $(packaging_targets):
 	$(MAKE) -C contrib/packaging $@
 
 osx:
 	rm -rf build/mercurial
 	/usr/bin/python2.7 setup.py install --optimize=1 \
 	  --root=build/mercurial/ --prefix=/usr/local/ \
 	  --install-lib=/Library/Python/2.7/site-packages/
 	make -C doc all install DESTDIR="$(PWD)/build/mercurial/"
         # Place a bogon .DS_Store file in the target dir so we can be
         # sure it doesn't get included in the final package.
 	touch build/mercurial/.DS_Store
         # install zsh completions - this location appears to be
         # searched by default as of macOS Sierra.
 	install -d build/mercurial/usr/local/share/zsh/site-functions/
 	install -m 0644 contrib/zsh_completion build/mercurial/usr/local/share/zsh/site-functions/_hg
         # install bash completions - there doesn't appear to be a
         # place that's searched by default for bash, so we'll follow
         # the lead of Apple's git install and just put it in a
         # location of our own.
 	install -d build/mercurial/usr/local/hg/contrib/
 	install -m 0644 contrib/bash_completion build/mercurial/usr/local/hg/contrib/hg-completion.bash
 	make -C contrib/chg \
 	  HGPATH=/usr/local/bin/hg \
 	  PYTHON=/usr/bin/python2.7 \
 	  DESTDIR=../../build/mercurial \
 	  PREFIX=/usr/local \
 	  clean install
 	mkdir -p $${OUTPUTDIR:-dist}
 	HGVER=$$(python contrib/genosxversion.py $(OSXVERSIONFLAGS) build/mercurial/Library/Python/2.7/site-packages/mercurial/__version__.py) && \
 	OSXVER=$$(sw_vers -productVersion | cut -d. -f1,2) && \
 	pkgbuild --filter \\.DS_Store --root build/mercurial/ \
 	  --identifier org.mercurial-scm.mercurial \
 	  --version "$${HGVER}" \
 	  build/mercurial.pkg && \
 	productbuild --distribution contrib/packaging/macosx/distribution.xml \
 	  --package-path build/ \
 	  --version "$${HGVER}" \
 	  --resources contrib/packaging/macosx/ \
 	  "$${OUTPUTDIR:-dist/}"/Mercurial-"$${HGVER}"-macosx"$${OSXVER}".pkg
 
 pyoxidizer:
 	$(PYOXIDIZER) build --path ./rust/hgcli --release
 
 .PHONY: help all local build doc cleanbutpackages clean install install-bin \
 	install-doc install-home install-home-bin install-home-doc \
 	dist dist-notests check tests rust-tests check-code format-c \
 	update-pot pyoxidizer \
 	$(packaging_targets) \
 	osx