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
# Balto (https://bitbucket.org/lothiraldan/balto/src) is a test orchestrator
# which is compatible with all test runner that can emit the LITF
# (https://github.com/lothiraldan/litf) test format.
# The plugin for the Mercurial test runner is mercurial-litf
# (https://pypi.org/project/mercurial-litf/). Make sure to follow the
# instruction and configuration instructions here:
# https://bitbucket.org/lothiraldan/mercurial_litf/src/default/
# You can launch Balto with `balto /path/to/mercurial/tests/`
name = "Mercurial Test Suite"
tool = "mercurial"