unpin pytest

[drammock]

cf. mne-tools/mne-python#12789

Locally for me all tests pass except for 3 symmetric_orth runs (2-False, 3-False, and 3-True). They pass on pytest 7.4.4 so this needs digging into

[larsoner]

Yes most pytest>=8 failures I ran into in moving other repos over was the need to have multiple pytest.warns context managers