Update to Python 3.9

[antirotor]

Feature

This PR is updating base OpenPype Python to 3.9 to match VFX platform CY2022

Close #3480

[ynbot]

Task linked: OP-3553 Update Python version to 3.9.x to match VFX platform CY2022

[antirotor]

first incompatibility:

Traceback (most recent call last):
  File "C:\Users\annat\.pyenv\pyenv-win\versions\3.9.6\lib\threading.py", line 973, in _bootstrap_inner
    self.run()
  File "C:\Users\annat\.pyenv\pyenv-win\versions\3.9.6\lib\threading.py", line 910, in run
    self._target(*self._args, **self._kwargs)
  File "C:\Users\annat\Documents\pype\op-p3.9\openpype\modules\ftrack\tray\ftrack_tray.py", line 207, in set_action_server
    elif not self.thread_socket_server.isAlive():
AttributeError: 'SocketThread' object has no attribute 'isAlive'

as isAlive() was removed in favour of is_alive()

[antirotor]

I believe this is actually testable. Tested in Maya 2020 so far without trouble....

[kalisp]

README.md in root needs to be updated too.

[antirotor]

Tested on Centos 7, need tests on Macs

[antirotor]

Should work on macs, tested on virtual Catalina

[64qam]

LGTM