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

contrib: comment out the 64-bit py38 dependency installation on Windows Not sure what is going on here, but it appears to not install py3.8 x64 in the usual `C:\hgdev` directory. The x32 installer works fine. I'm assuming this is a bug in this version of the installer, but didn't look into it too much. Differential Revision: https://phab.mercurial-scm.org/D11274

contrib: drop python2.7 from the Windows dependency install script While we aren't quite ready to drop py27 yet, the MS compiler is no longer available from MS (there is a copy on github if it's really needed), and that causes the script to fail. Differential Revision: https://phab.mercurial-scm.org/D11273