appveyor: Use Python3.

This implied upgrading to the Visual Studio 2019 image, not for VS
itself, but for the newer Python 3.8.5 version it contains, to avoid
UnicodeDecodeError inside modulefinder module when attempting to decode
our UTF-8 encoded Python scripts with cp1252 encoding.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6184>
2 files changed