Note: most of the foundational work of this commit was done by Matt Harbison,
but enough has changed that I don't feel comfortable sending this patch as his.
In our quest to remove Python 2 from Mercurial, we need to get Windows CI going
to check that Python 3 support is up to an acceptable standard.
This is the first step: adding a manual step to run a full Windows CI, with a
certain definition of "full" since some things are not installed yet, like SVN.