Remove obsolete `mock` dependency

[mgorny]

Remove the dependency on mock package, as the test suite uses only the builtin unittest.mock module.

Checklist:

• [ ] Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• [ ] Add or update relevant docs, in the docs folder and in code.
• [ ] Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• [ ] Add .. versionchanged:: entries in any relevant code docs.
• [ ] Run pre-commit hooks and fix any issues.
• [x] Run pytest and tox, no tests failed.

[mgorny]

The CI failures don't seem related to my change.

[pgjones]

Thanks

[mgorny]

Thanks!