GitHub Action to replace Travis CI

[cclauss]

Test results: https://github.com/cclauss/python-magic/actions

The Python 3.13 release notes mention python-magic 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

[cclauss]

@ahupp Your review, please. It would help to have automated tests running.