Add pytest-cov to dev dependency group (#846)

The shared pytest addopts include --cov=numcodecs, so every dependency
group that runs pytest needs pytest-cov. After the test-zarr-* groups
were deduped to include the dev group, pytest-cov was no longer pulled
in, breaking external runners (e.g. zarr-python's numcodecs codec tests
job) with "unrecognized arguments: --cov=numcodecs".

Put pytest-cov in dev so all groups that include it get coverage support.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: d-v-b

pyproject.toml

@@ -88,6 +88,7 @@ dev = [
     "setuptools-scm>=6.2",
     "numpy>=2",
     "pytest==9.0.3",
+    "pytest-cov",
 ]
 test-zarr-308 = [
     {include-group="dev"},