Use platformdirs package to use platform-dependent user cache directory

[matthewfeickert]

scikit-hep-testdata/src/skhep_testdata/data/__init__.py

Line 8 in 8c71761

skhepdir = Path.home() / ".local" / "skhepdata"

We could use https://pypi.org/project/appdirs/ to discover the platform-dependent user_cache_dir

Originally posted by @nsmith- in #197 (comment)

https://github.com/ActiveState/appdirs/blob/8734277956c1df3b85385e6b308e954910533884/README.rst?plain=1#L4

Note: This project has been officially deprecated. You may want to check out https://pypi.org/project/platformdirs/ which is a more active fork of appdirs

Though https://github.com/tox-dev/platformdirs has no dependencies and is part of the Tox ecosystem of tools, so that sounds reasonable.