Cleanup Python 3.8 leftovers

[V02460]

Some small cleanups after Python3.8 became EOL.

• Move some type imports from typing_extensions to typing
• Remove the abi3-py38 feature from pyo3

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct
  (run the linters)

[V02460]

I added the abi3-py39 option, so this PR can be merged. Should I find out it is better omitted, I’ll open a new PR.