makefile: use Python 3 by default when building docs as well

I copied the snippet defining `PYTHON` from the root Makefile.

Differential Revision: https://phab.mercurial-scm.org/D10251

doc/Makefile

@@ -1,48 +1,55 b''
 SOURCES=$(notdir $(wildcard ../mercurial/helptext/*.[0-9].txt))
 MAN=$(SOURCES:%.txt=%)
 HTML=$(SOURCES:%.txt=%.html)
 GENDOC=gendoc.py ../mercurial/commands.py ../mercurial/help.py \
 	../mercurial/helptext/*.txt ../hgext/*.py ../hgext/*/__init__.py
 PREFIX=/usr/local
 MANDIR=$(PREFIX)/share/man
 INSTALL=install -m 644
-PYTHON?=python
+# Default to Python 3.
+#
+# Windows ships Python 3 as `python.exe`, which may not be on PATH.  py.exe is.
+ifeq ($(OS),Windows_NT)
+PYTHON?=py -3
+else
+PYTHON?=python3
+endif
 RSTARGS=
 
 export HGENCODING=UTF-8
 
 all: man html
 
 man: $(MAN)
 
 html: $(HTML)
 
 # This logic is duplicated in setup.py:hgbuilddoc()
 common.txt $(SOURCES) $(SOURCES:%.txt=%.gendoc.txt): $(GENDOC)
 	${PYTHON} gendoc.py "$(basename $@)" > $@.tmp
 	mv $@.tmp $@
 
 %: %.txt %.gendoc.txt common.txt
 	$(PYTHON) runrst hgmanpage $(RSTARGS) --halt warning \
 	  --strip-elements-with-class htmlonly $*.txt $*
 
 %.html: %.txt %.gendoc.txt common.txt
 	$(PYTHON) runrst html $(RSTARGS) --halt warning \
 	  --link-stylesheet --stylesheet-path style.css $*.txt $*.html
 
 MANIFEST: man html
 # tracked files are already in the main MANIFEST
 	$(RM) $@
 	for i in $(MAN) $(HTML); do \
 	  echo "doc/$$i" >> $@ ; \
 	done
 
 install: man
 	for i in $(MAN) ; do \
 	  subdir=`echo $$i | sed -n 's/^.*\.\([0-9]\)$$/man\1/p'` ; \
 	  mkdir -p "$(DESTDIR)$(MANDIR)"/$$subdir ; \
 	  $(INSTALL) $$i "$(DESTDIR)$(MANDIR)"/$$subdir ; \
 	done
 
 clean:
 	$(RM) $(MAN) $(HTML) common.txt $(SOURCES) $(SOURCES:%.txt=%.gendoc.txt) MANIFEST