Drop Python 3.8 and 3.9 support

[rustaceanrob]

Python 3.9 is 5 years old. The CI builds the Python binary for every platform and every version from 3.8-latest, but hasn't caught a significant issue.

Motivated by #769

Changelog notice

• CI no longer builds for Python 3.8 and 3.9

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[ItoroD]

ACK 5939076

[reez]

Makes sense, talked about it together yesterday at the Bindings call.

tl;dr:

• give folks this week to do any last review/feedback
  • ask @andreasgriffin (might have been asked already, but asking him on the PR) if he's cool w it (assume he is)
• double check other places we use/reference 3.8/3.9 in the codebase other than this, and remove too if needed (can be done by anyone in a follow up PR)

[andreasgriffin]

Fine by me 👍

[reez]

ACK 693ccb9

We can merge this next week, after rebase (which I can even do if needed, nbd), no rush, and I've got a small follow up PR that removes a couple other 3.8/3.9

Thanks for opening this PR 👍

[rustaceanrob]

Rebased