Python 3.5 no longer supported [by Python, that is]; 3.9.0 released

[GriceTurrble]

Python has officially dropped support for 3.5. See Python Insider announcement, posted Oct 2: https://pythoninsider.blogspot.com/2020/10/python-35-is-no-longer-supported.html

Update Oct 5: Python 3.9.0 stable has been released: https://www.python.org/downloads/release/python-390/

PAM impact

We'll be dropping tests for 3.5 from the CI test suite on next major update (after #207 merge). Users are recommended to upgrade their applications to the latest Python at earliest convenience.

On the positive note, after we officially drop 3.5 support in the test suite, we can begin accepting changes that utilize features added in 3.6, such as f-strings. We can also accept type hinting syntax, which we've been avoiding to date.

Update Oct 5: per the release of Python 3.9.0, we will add that to the test suite shortly, as soon as I see GitHub actions supports it (3.9.0rc1 is already set on the current major PR, so I'll be swapping that).

No specific timeline on these changes, just wanted folks to be aware. Thank you!