build!: update supported Python versions

[TomJGooding]

Python 3.14 should be released in just a few weeks, which means Python 3.8 has been end-of-life for almost a year.

This drops support for Python 3.8 and adds testing in CI for Python 3.14.

I expect tests will fail on Python 3.14 until this issue is resolved:

• calls asyncio.get_event_loop() in a way that will fail on 3.14 #45

[willmcgugan]

Thanks for doing the legwork! I've addressed the remaining issues.