Bump coverage from 7.8.2 to 7.9.1 in /requirements

Bumps [coverage](https://github.com/nedbat/coveragepy) from 7.8.2 to 7.9.1.
- [Release notes](https://github.com/nedbat/coveragepy/releases)
- [Changelog](https://github.com/nedbat/coveragepy/blob/master/CHANGES.rst)
- [Commits](nedbat/coveragepy@7.8.2...7.9.1)

---
updated-dependencies:
- dependency-name: coverage
  dependency-version: 7.9.1
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

requirements/tests.txt

@@ -1,5 +1,5 @@
 -r dev.txt
-coverage==7.8.2
+coverage==7.9.1
 requests-mock==1.12.1
 tblib>=3.0.0
 tox<5.0.0