build(python)!: Drop Python 3.7 support

[stinodego]

!! THIS IS A BREAKING CHANGE !!

Re-opened from #6824

Changes:

• Set required Python version to >=3.8
• Set PyO3 abi3 feature to Python 3.8
• Update CI to run on Python 3.8 instead of 3.7 (for release, test, and mypy jobs).
• Update some typing imports
• Remove some typing workarounds
• Update pandas-stubs version and update some pandas type hints

Notes:

• We still must allow untyped calls for mypy on Python 3.8 due to the backports.zoneinfo package having some issues.

I'd like to have this part of the 0.17.0 release, but if you prefer we can keep supporting Python 3.7 until end of life (27 June 2023). In that case we'd probably merge this with the 1.0.0 release of Polars.

[ritchie46]

I really want to be a bit conservative here. Python 3.7 is our biggest share in downloads still. https://pypistats.org/packages/polars

[stinodego]

Interesting! Definitely, we should wait with this for now then. Let's re-assess in the 1.0.0 release 👍

[stinodego]

Rebasing this on a new branch.