Drop python 3.9 support

[dperl-dls]

According to NEP-29 we can now drop python 3.9 🎉
This makes many nice 3.10 features available, such as ParamSpec and getting rid of Optional and Union for |.

[evalott100]

Looks good to rebase off main and merge, boy I can't wait to use match statements again 🙏 😭

[dperl-dls]

@evalott100 What's with these tests which never run? Are they expected by some repo settings?

[evalott100]

@evalott100 What's with these tests which never run? Are they expected by some repo settings?

They're for the different situations - i.e on a "release" the release ones will be ran...

It's strange you have two pending tests not categorised like the others, not sure why this would be happening

[evalott100]

Somehow this has introduced these:

[dperl-dls]

I think they are required by some repo settings not in the code - as you can see with the latest commit, running the 3.10 tests with editable install satisfied the expectation for that - while the 3.9 one is not satisfied because it is no longer being run, because it is removed from the actions. I don't have permissions to update the branch protection rules here, some admin needs to do that - @coretl ?

[coretl]

done

Branch admin