debian: support building a single deb for multiple py3 versions

Around transitions from one python minor version to another (such as 3.7 to
3.8), the current packaging can be slightly problematic - it produces a
`control` file that requires that the version of `python3` that's installed be
exactly the one that was used on the build machine for the `mercurial` package,
by containing a line like:

    Depends: sensible-utils, libc6 (>= 2.14), python3 (<< 3.8), python3 (>= 3.7~), python3:any (>= 3.5~)

This is because it "knows" we only built for v3.7, which is the current default
on my system. By building the native components for multiple versions, we can
make it produce a line like this, which is compatible with 3.7 AND 3.8:

    Depends: sensible-utils, libc6 (>= 2.14), python3 (<< 3.9), python3 (>= 3.7~), python3:any (>= 3.5~)

This isn't *normally* required, so I'm not making it the default. For those that
receive their python3 and mercurial packages from their distro, and/or don't
have to worry about a situation where the team that manages the python3
installation isn't the same as the team that manages the mercurial installation,
this is probably not necessary.

I chose the names `DEB_HG_*` because `DEB_*` is passed through `debuild`
automatically (otherwise we'd have to explicitly allow the options through,
which is a nuisance), and the `HG` part is to make it clear that this isn't a
"standard" debian option that other packages might respect.

Test Plan:
1. "nothing changed":
    - built a deb without these changes
    - built a deb with these changes but everything at the default
    - used diffoscope to compare, all differences were due to timestamps
2. "explicit is the same as implicit" (single version)
    - built a deb with everything at the default
    - built a deb with DEB_HG_PYTHON_VERSIONS=3.7
    - used diffoscope to compare, all differences were due to timestamps
3. "explicit is the same as implicit" (multi version)
    - built a deb with DEB_HG_MULTI_VERSION=1
    - built a deb with DEB_HG_PYTHON_VERSIONS=3.7
    - used diffoscope to compare, all differences were due to timestamps
4. (single version, 3.7) doesn't work with python3.8
    - `/usr/bin/python3.7 /usr/bin/hg debuginstall` works
    - `/usr/bin/python3.8 /usr/bin/hg debuginstall` crashes
5. (multi version, 3.7 + 3.8)
    - `/usr/bin/python3.7 /usr/bin/hg debuginstall` works
    - `/usr/bin/python3.8 /usr/bin/hg debuginstall` works

Differential Revision: https://phab.mercurial-scm.org/D8642

contrib/packaging/debian/rules

@@ -2,14 +2,51 b''
 # Uncomment this to turn on verbose mode.
 # export DH_VERBOSE=1
 
+# By default we build a .deb where the native components are built with the
+# current "default" version of py3 on the build machine. If you wish to build a
+# .deb that has native components built for multiple versions of py3:
+#
+#   1. install python3.x and python3.x-dev for each version you want
+#   2. set DEB_HG_MULTI_VERSION=1 or DEB_HG_PYTHON_VERSIONS in your environment
+#      (if both are set, DEB_HG_PYTHON_VERSIONS has precedence)
+#
+# If you choose `DEB_HG_MULTI_VERSION=1`, it will build for every "supported"
+# version of py3 that's installed on the build machine. This may not be equal to
+# the actual versions that are installed, see the comment above where we set
+# DEB_HG_PYTHON_VERSIONS below. If you choose to set `DEB_HG_PYTHON_VERSIONS`
+# yourself, set it to a space-separated string of python version numbers, like:
+#   DEB_HG_PYTHON_VERSIONS="3.7 3.8" make deb
+DEB_HG_MULTI_VERSION?=0
+
 CPUS=$(shell cat /proc/cpuinfo | grep -E ^processor | wc -l)
 
+# By default, only build for the version of python3 that the system considers
+# the 'default' (which should be the one invoked by just running 'python3'
+# without a minor version). If DEB_HG_PYTHON_VERSIONS is set, this is ignored.
+ifeq ($(DEB_HG_MULTI_VERSION), 1)
+	# If we're building for multiple versions, use all of the "supported" versions
+	# on the build machine. Note: the mechanism in use here (`py3versions`) is the
+	# recommended one, but it relies on a file written by the python3-minimal
+	# package, and this file is not dynamic and does not account for manual
+	# installations, just the ones that would be installed by `python3-all`. This
+	# includes the `-i` flag, which claims it's to list all "installed" versions,
+	# but it doesn't. This was quite confusing, hence this tale of woe. :)
+	DEB_HG_PYTHON_VERSIONS?=$(shell py3versions -vs)
+else
+	# If we're building for only one version, identify the "default" version on
+	# the build machine and use that when building; this is just so that we don't
+	# have to duplicate the rules below for multi-version vs. single-version. The
+	# shebang line will still be /usr/bin/python3 (no minor version).
+	DEB_HG_PYTHON_VERSIONS?=$(shell py3versions -vd)
+endif
+
 export HGPYTHON3=1
 export PYTHON=python3
 
 %:
 	dh $@ --with python3
 
+# Note: testing can be disabled using the standard `DEB_BUILD_OPTIONS=nocheck`
 override_dh_auto_test:
 	http_proxy='' dh_auto_test -- TESTFLAGS="-j$(CPUS)"
 
@@ -24,8 +61,15 b' override_dh_auto_build:'
 	$(MAKE) all
 	$(MAKE) -C contrib/chg all
 
-override_dh_auto_install:
-	python3 setup.py install --root "$(CURDIR)"/debian/mercurial --install-layout=deb
+# Build the native extensions for a specfic python3 version (which must be
+# installed on the build machine).
+install-python%:
+	python$* setup.py install --root "$(CURDIR)"/debian/mercurial --install-layout=deb
+
+# Build the final package. This rule has a dependencies section that causes the
+# native extensions to be compiled for every version of python3 listed in
+# DEB_HG_PYTHON_VERSIONS.
+override_dh_auto_install: $(DEB_HG_PYTHON_VERSIONS:%=install-python%)
 	# chg
 	make -C contrib/chg \
 		DESTDIR="$(CURDIR)"/debian/mercurial \