Bump mypy from 0.920 to 0.921 in /requirements

Bumps [mypy](https://github.com/python/mypy) from 0.920 to 0.921.
- [Release notes](https://github.com/python/mypy/releases)
- [Commits](python/mypy@v0.920...v0.921)

---
updated-dependencies:
- dependency-name: mypy
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

requirements/dev-requirements.txt

@@ -15,7 +15,7 @@ freezegun==1.1.0  # Freeze time in tests
 furo==2021.11.23
 isort==5.10.1  # Lint imports
 keyring==23.4.0
-mypy==0.920 # Type checking
+mypy==0.921 # Type checking
 pip_check_reqs==2.3.2
 pydocstyle==6.1.1  # Lint docstrings
 pyenchant==3.2.2  # Bindings for a spellchecking sytem