What kind of feature are you missing? Where do you notice a shortcoming of PTB?

Python 3.10 was released very recently. While we won't be able to add any of its new features without breaking backward compatibility, we can add to our Github Actions test matrix and update the README to say the same.

Describe the solution you'd like

• Update both readme's to say we support python 3.10
• Update setup.py
• Add python 3.10 to github action test matrix.

Describe alternatives you've considered

No response

Additional context

A PR for this should be based on v14. see #2681