chore(deps): bump types-six from 1.16.18 to 1.16.21 in /dev_requirements (#338)

Bumps [types-six](https://github.com/python/typeshed) from 1.16.18 to 1.16.21.
- [Release notes](https://github.com/python/typeshed/releases)
- [Commits](https://github.com/python/typeshed/commits)

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

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Darwin Chowdary <[email protected]>

Author: dependabot[bot]

dev_requirements/coverage-requirements.txt

@@ -3,4 +3,4 @@ mypy==0.971
 mypy-extensions==0.4.3
 typing==3.7.4.3
 types-setuptools==63.2.2
-types-six==1.16.18
+types-six==1.16.21