Replace appdirs with platformdirs

[manselmi]

Summary

Replace the appdirs dependency with platformdirs, an actively-maintained fork.

Also, declare platformdirs as a dependency in pyproject.toml.

Details

Today I used pystow for the first time, and encountered a ModuleNotFoundError after setting the environment variable PYSTOW_USE_APPDIRS to true:

% env -- PYSTOW_USE_APPDIRS=true python -c 'import pystow'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
    import pystow
  File "/Users/manselmi/virtualenv/pystow-py313/lib/python3.13/site-packages/pystow/__init__.py", line 101, in <module>
    ensure_readme()
    ~~~~~~~~~~~~~^^
  File "/Users/manselmi/virtualenv/pystow-py313/lib/python3.13/site-packages/pystow/utils.py", line 1070, in ensure_readme
    readme_path = get_home(ensure_exists=True).joinpath("README.md")
                  ~~~~~~~~^^^^^^^^^^^^^^^^^^^^
  File "/Users/manselmi/virtualenv/pystow-py313/lib/python3.13/site-packages/pystow/utils.py", line 1029, in get_home
    from appdirs import user_data_dir
ModuleNotFoundError: No module named 'appdirs'

I hadn't noticed the warning in docs/source/installation.rst about needing to manually install appdirs, as I had looked only at the top-level README, which didn't contain a similar warning. The desire to avoid manual installation of appdirs motivated me to submit a PR to declare it as a dependency in pyproject.toml.

However, given that appdirs is deprecated and platformsdirs is a suggested replacement, I figured I should take this opportunity to replace appdirs with platformdirs.

Now, the user may set the environment variable PYSTOW_USE_PLATFORMDIRS (or
PYSTOW_USE_APPDIRS for backwards compatibility) to opt-in to the platform-specific user data directory.

[manselmi]

Edited summary to include demonstration of import pystow failing when PYSTOW_USE_APPDIRS is true