Update Python support to 3.10+

[tim-schilling]

• Drops support for Python 3.9 and below
• Adds support for 3.14
• Removes the backports and shims
• Replaces nose for pytest for test runner

Sorry for the duplicate PR. I dropped the other change around setting a specific encoding as it seemed silly on re-review.

[timgraham]

Could you please send some of the clean up work as separate PRs? For example, I see Python 2 removals in the "Update Python support to 3.10+" commit. Also, PR checks have not run on this PR. I'm not sure if a change here broken them or if there's an existing issue.

[tim-schilling]

Hi @timgraham 👋 I think I've extracted out the modernizations from that commit. Though I disagree with you about splitting it up into multiple PRs. When the maintainer comes around, they can decide if they want to split things up or merge them all in one go. Hard to know what they'll want right now. I'm also happy to have someone else cherry-pick the commits or redo things. I was just trying to help things along.

As for why the tests aren't running, it's because the action is triggered on push and my PR would require pull_request, but mine wouldn't run without maintainer approval anyway.

[timgraham]

I have merge permissions on this repo. I don't see why tests shouldn't run on PRs, so I have proposed it in #199.