Add support for Python 3.9

[osrjv]

It seems that some dependencies are lacking pre-built wheels for Python 3.9. Check which ones and upgrade as necessary.

Reported originally on forums: https://forum.robocorp.com/t/experimental-library-tasks/51/2

[osrjv]

Added workflow for checking Python 3.9: https://github.com/robocorp/rpaframework/actions?query=workflow%3Arpaframework-py39

At least these are issues:

• pylint seems to be creating false positives, known bug: [Python 3.9] Value 'Optional' is unsubscriptable (unsubscriptable-object) (also Union) pylint-dev/pylint#3882
• cryptography is missing a wheel for Windows with the current version

[osrjv]

Python 3.9 was officially added to our CI test matrix for release 10.0.0