DEP: Can we drop support for Python 3.9 ?

[neutrinoceros]

I'd like to propose dropping support for Python 3.9 now that we're about to add explicit support for 3.13.
My main motivation is that I'd like to have access to the match/case statement to implement support for np.arange(..., like=unyt_array(...)) which is part of #518. For reference, see my implementation for astropy.units in astropy/astropy#17059

Thoughts, @jzuhone ?

[neutrinoceros]

Denver discussed this privately with @jzuhone and agreed to do this after unyt 3.0.4, which is in preparation.