fix: Remove pkg_resources

[BenjaminHalko]

pkg_resources is depreciated in Python 3.12. This PR replaces it with importlib.resources.

[craffel]

Thanks, but pkg_resources is still used to get the default SF2 path:

pretty-midi/pretty_midi/fluidsynth.py

Line 52 in 280352a

synthesizer = pkg_resources.resource_filename(__name__, DEFAULT_SF2)

That would need to be fixed (and we'd need to verify that, after applying this PR, the library could be installed with the default SF2 in place and usable by the fluidsynth code).