contrib: switch the Windows bootstrap environment to py3.9
Use the built in `venv` module instead of `virtualenv` for simplicity, and
upgrade to a modern Mercurial that supports py3.
One issue here is that `venv` doesn't copy `python3{,Y}.dll` into the `Scripts`
subdirectory, so running the `hg.exe` that gets installed immediately fails on a
clean system because Python isn't in `PATH`. There is code in `python.exe` to
detect when it is in a venv and add the original python install to the DLL
lookup path, which we don't do in `hg.exe` yet. The simple workaround for now is
to run the `hg` script with `python.exe`. Typically `PYTHONLEGACYWINDOWSSTDIO`
must be set in the environment on Windows, but the clone process works without
it.
Differential Revision: https://phab.mercurial-scm.org/D11275
#require docutils gettext
Error: the current ro localization has some rst defects exposed by
moving pager to core. These two warnings about references are expected
until the localization is corrected.
$ $TESTDIR/check-gendoc ro
checking for parse errors
gendoc.txt:58: (WARNING/2) Inline interpreted text or phrase reference start-string without end-string.
gendoc.txt:58: (WARNING/2) Inline interpreted text or phrase reference start-string without end-string.