Upgrade Windows CI Python to 3.8 (#59729)
Summary:
Python 3.6 EOL is end of this year--we should use newer Python in CI.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59729
Reviewed By: bdhirsh
Differential Revision: D29006807
Pulled By: janeyx99
fbshipit-source-id: c79214b02a72656058ba5d199141f8838212b3b6
