build: drop support for Python 3.8, add CI checks for 3.13

[mikix]

3.8 recently went end-of-life and 3.13 was released.

• Testing 3.13 is a no-brainer, especially since it removes a bunch of deprecated modules that we want to be sure we aren't using.
• Removing 3.8 makes our life easier and is in line with our recent removal of python2 and <3.8 as python versions go end-of-life. Previous releases of fhirclient will still be pip-installable for 3.8.

The funnest new thing in 3.9 that we might consider using more heavily is "type hinting generics in standard collections".

i.e. saying list[str] instead of List[str]

But the current code base doesn't have much hinting at all, so no changes needed yet. But we can use that going forward.