API: Return Path in fetch_fsaverage

[larsoner]

Closes #12768

Transitioned some testing and datasets code to pathlib.Path while I was in there tinkering anyway.

[drammock]

test needs an os.sep somewhere. TLDR fsaverage/label != fsaverage\\label:

mne\datasets\tests\test_datasets.py:91: in test_datasets_basic
    assert sd_label_str == str(sd_2 / "label")
E   AssertionError: assert 'C:\\Users\\runneradmin\\AppData\\Local\\Temp\\pytest-of-runneradmin\\pytest-0\\test_datasets_basic0\\mne_data\\MNE-fsaverage-data\\fsaverage/label' == 'C:\\Users\\runneradmin\\AppData\\Local\\Temp\\pytest-of-runneradmin\\pytest-0\\test_datasets_basic0\\mne_data\\MNE-fsaverage-data\\fsaverage\\label'

[hoechenberger]

thanks!