Abandon python3.8

[hemidactylus]

This PR introduces Python3.9 as the minimum required version.

As a matter of fact, the code is already not usable on 3.8 due to the tuple[...] syntax recently introduced: attempting import cassio results in:

  File ".../cassio/src/cassio/config/__init__.py", line 41, in <module>
    ) -> tuple[Session, Optional[str]]:
TypeError: 'type' object is not subscriptable

No need to support 3.8 in future versions anyway.