CI crashing because marks applied to fixtures now raise errors in newer versions of pytest

[lewisblake]

Many tests on CI currently show something like the following, where the GAW tests are failing:

Run pytest -ra -q --cov --no-cov-on-fail --cov-report xml

==================================== ERRORS ====================================
________________ ERROR collecting tests/plugins/gaw/test_dms.py ________________
tests/plugins/gaw/test_dms.py:16: in <module>
    def data_vmrdms_ams_cvo():
/usr/share/miniconda/envs/pya/lib/python3.10/site-packages/_pytest/fixtures.py:1212: in __call__
    warnings.warn(MARKED_FIXTURE, stacklevel=2)
E   pytest.PytestRemovedIn9Warning: Marks applied to fixtures have no effect
E   See docs: https://docs.pytest.org/en/stable/deprecations.html#applying-a-mark-to-a-fixture-function
=========================== short test summary info ============================
SKIPPED [1] tests/plugins/icos/test_reader.py:11: Skipping tests that require access to AEROCOM database on METNo servers
ERROR tests/plugins/gaw/test_dms.py - pytest.PytestRemovedIn9Warning: Marks applied to fixtures have no effect
See docs: https://docs.pytest.org/en/stable/deprecations.html#applying-a-mark-to-a-fixture-function
!!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!
1 skipped, 1 error in 2.80s
Error: Process completed with exit code 2.

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