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

contrib: log the command and args for every process installing windows deps This is a little noisier, but makes it simple to debug when things fail. Differential Revision: https://phab.mercurial-scm.org/D11272

windows-ci: run Windows CI automatically alongside the others This will enable us to make Windows Python 3 a first-class citizen for the next 6.0 cycle. We will probably get some flaky tests and we're missing others that are skipped, but we'll turn them on it future patches. Differential Revision: https://phab.mercurial-scm.org/D11256

vfs: always use / as file separator (issue6546) Various part of vfs already enforce `/` usage and using `\` confuse the encoded vfs. So we simply use `/` all the time. Differential Revision: https://phab.mercurial-scm.org/D11260

subrepo: compare normalised vfs path Otherwise the realpath call can turn `/` into `\` on windows confusing the check. (We probably needs this in more location) Differential Revision: https://phab.mercurial-scm.org/D11259