Python 3.9

[hugovk]

Python 3.9.0 was released yesterday, 2020-10-05:

• https://discuss.python.org/t/python-3-9-0-is-now-available-and-you-can-already-test-3-10-0a1/5375?u=hugovk

Please could you also add support in AppVeyor for Python 3.9?

Thank you!

PS It took 25 days for 3.8: #3142 :)