Drop Python 3.9 support

[ezio-melotti]

This PR drops Python 3.9 support.

There are two reasons for this:

• Python 3.9 recently reached EOL;
• coverage now requires Python 3.10 (see #433)

Previous PR here:

• #407

Before merging we also need an admin (@Mariatta?) to remove the 3.9 checks from the branch settings.