automation: support running against Python 3.9...
automation: support running against Python 3.9
We just added CPython 3.9.0 to the automation environment. Let's
support actions targeting it.
One side-effect of this change is that we'll produce Windows wheels
for Python 3.9 by default. This is desirable.
Differential Revision:
https://phab.mercurial-scm.org/D9189