GitHub Actions: Test on Python 3.13 beta

[cclauss]

The Python 3.13 release notes mention puremagic as one of the alternatives for imghdr which was removed from the Standard Library so let's ensure that its tests pass on Python 3.13 beta.

https://www.python.org/downloads/release/python-3130b1/

May raise ModuleNotFoundError: No module named 'imghdr' because Python 3.13 removes it from the Standard Library.

• https://docs.python.org/3/library/imghdr.html

imghdr: use the projects filetype, puremagic, or python-magic instead. (Contributed by Victor Stinner in gh-104773.)

https://docs.python.org/3.13/whatsnew/3.13.html#pep-594-dead-batteries-and-other-module-removals

[NebularNerd]

@NebularNerd Do you have any insight into why the two Ubuntu tests fail?

Hi @cclauss I got an email with the above question but it's not showing up in this PR when I view it, I've cut'n'pasted it from the mail. I'm not sure I can help on that, I'm still fairly new to GitHub and have not played much with the workflow side of things.

@cdgriffith the repo owner will be the best person to take a look.