Allow Python 3.13

[befeleme]

I've built pypandoc successfully with Python 3.13.0a5 in Fedora Linux 41 (and all the tests passed), so I think it could be good to relax the supported Python version range.

[JessicaTegner]

@befeleme thanks for the pr :)

If it's working, would it be possible for you to update the CI script, to also test for python 3.13?

[branchvincent]

Bump on this, 3.13 was released a couple weeks ago and the <3.13 cap blocks dependents like https://github.com/Aider-AI/aider

Would you also consider removing the upper bound entirely? See https://iscinumpy.dev/post/bound-version-constraints/#pinning-the-python-version-is-special for a discussion on why the general recommendation is to leave it unbounded

[JessicaTegner]

Hey @befeleme and @branchvincent #379 will superset this pr. @branchvincent do let me know in there, if this is what you were looking after.