Test Python 3.8 and PyPy3.6 on Windows with GitHub Actions

[nulano]

Adds Python 3.8 to Windows build matrix on GitHub Actions.
Edit: Also adds PyPy3.6-7.2.0 to Windows build matrix on GitHub Actions.

[hugovk]

Thanks!

[nulano]

I finally managed to get PyPy to build - after comparing traces in a debugger, I discovered that unlike CPython3.6 (which uses distutils._msvccompiler), PyPy uses distutils.msvc9compiler, which requires theMSSdk envar in addition to DISTUTILS_USE_SDK.

Edit: Link to relevant discussion: #4188, #4201

[hugovk]

Thanks again!