Un-blacklist p4python on Python 3.9.
Review Request #11845 — Created Oct. 7, 2021 and submitted
p4python now has builds for Python 3.9, so we can un-blacklist it.
Ran setup.py develop for 3.9 and saw that it didn't try to brokenly
As of 2021.1.2128019. I actually have almost this same change in my tree (along with other Python 2 removals — …