Adds alternate download approach for nuget.exe
Fall back to git.exe if no Python is found. (#2739)
Also check whether git.exe is on PATH if it will be used.
Add support for HOST_PYTHON variable.
Clear internal environment variables used in find_python.bat
Use HOST_PYTHON as the actual Python if it is recent enough.
Adds HOST_PYTHON variable to AppVeyor configuration
The Windows build now depends on Python 3.6 to fetch externals, but it will be downloaded via NuGet (which is downloaded via PowerShell) if it is not available via `py -3.6`. This means the only thing that must be installed on a modern Windows box to do a full build of CPython with all extensions is Visual Studio.
Also fixes an outdated note about _lzma in PCbuild/readme.txt
(cherry-picked from commit 51599e2bdd)
Steve Dower
Zachary Ware