CI: (deps): Bump coverage from 7.6.7 to 7.6.8 in /ci

Bumps [coverage](https://github.com/nedbat/coveragepy) from 7.6.7 to 7.6.8.
- [Release notes](https://github.com/nedbat/coveragepy/releases)
- [Changelog](https://github.com/nedbat/coveragepy/blob/master/CHANGES.rst)
- [Commits](nedbat/coveragepy@7.6.7...7.6.8)

---
updated-dependencies:
- dependency-name: coverage
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

ci/test_requirements.txt

@@ -2,4 +2,4 @@ packaging==24.2
 pytest==8.3.3
 pytest-mpl==0.17.0
 netCDF4==1.7.2
-coverage==7.6.7
+coverage==7.6.8