BUG: wrong dependencies for pooch

[tupui]

Pooch made an update which removes the deps to appdirs. Apparently we should update to platformdirs. See fatiando/pooch#339

It means our CI is failing with: https://dev.azure.com/scipy-org/SciPy/_build/results?buildId=23474&view=logs&jobId=ede89041-5e8e-5201-ce5c-f68c2cbf02dd&j=ede89041-5e8e-5201-ce5c-f68c2cbf02dd&t=bf6d2fae-c4b4-5aaf-e29f-d3092465da33

scipy.datasets.clear_cache
--------------------------

File "build/testenv/lib/python3.9/site-packages/scipy/datasets/_utils.py", line 78, in clear_cache
Failed example:
    datasets.clear_cache([datasets.ascent])
Exception raised:
    Traceback (most recent call last):
      File "/opt/hostedtoolcache/Python/3.9.16/x64/lib/python3.9/doctest.py", line 1334, in __run
        exec(compile(example.source, filename, "single",
      File "<doctest clear_cache[3]>", line 1, in <module>
        datasets.clear_cache([datasets.ascent])
      File "/home/vsts/work/1/s/build/testenv/lib/python3.9/site-packages/scipy/datasets/_utils.py", line 81, in clear_cache
        _clear_cache(datasets)
      File "/home/vsts/work/1/s/build/testenv/lib/python3.9/site-packages/scipy/datasets/_utils.py", line 19, in _clear_cache
        raise ImportError("Missing optional dependency 'pooch' required "
    ImportError: Missing optional dependency 'pooch' required for scipy.datasets module. Please use pip or conda to install 'pooch'.

cc @AnirudhDagar