Use Python3 on Appveyor builds.
/cc @eric @pal1000 @brianp @charmainel @sroland @bhenden @kusma
SCons builds work fine with Python 3, provided one uses a recent enough Python 3 version --https://ci.appveyor.com/project/jrfonseca/mesa/builds/34489918
So there's no need to hold back on Python 2 deprecation on account of SCons.