Provide pytest-plugin that registers a 'gpu' marker.

[TomAugspurger]

https://github.com/zarr-developers/zarr-python/pull/3191/files#diff-a801b931f5dd8cd7df666e3f9936ff520669296303d3a3e469514443211885bdR43 changed zarr.testing.utils to use a top-level pytest.mark.gpu. Many projects, including zarr, run with --strict-markers which causes pytest to raise an error if the test you're running uses an unregistered marker. This didn't cause any issues for zarr, since we already registered the gpu marker.

However, downstream projects are intended to run the StoreTests, which exposes them to this marker. To ensure they get it when installing zarr, we'll include an entrypoint with pytest, so that pytest calls our code to register this marker when these tests run.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.59%. Comparing base (9969a5d) to head (13bcf1b).
⚠️ Report is 98 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3258       +/-   ##
===========================================
- Coverage   94.62%   59.59%   -35.04%     
===========================================
  Files          78       78               
  Lines        8696     8696               
===========================================
- Hits         8229     5182     -3047     
- Misses        467     3514     +3047     

see 66 files with indirect coverage changes

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