Drop Python 3.8 support

[vkbo]

Summary:

Since we're no longer supporting Ubuntu 20.04 with native packages from 2.5 release, due to requiring Qt 5.15, we may as well drop Python 3.8 as well as it is causing issues with type annotations. Ubuntu 20.04 uses Python 3.8, but 22.04 has Python 3.10. We could in principle drop Python 3.9 as well, but at the moment it isn't necessary.

Related Issue(s):

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All flake8 checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed