Drop Python 3.7 support

Python 3.7 is now end-of-life

The last security-only release of Python 3.7 was on 6th June 2023:
https://peps.python.org/pep-0537/#schedule-last-security-only-release

> The code base for 3.7 is now frozen and no further updates will be
> provided nor issues of any kind will be accepted on the bug tracker.