Bump mypy from 0.961 to 0.991 (#176)

Bumps [mypy](https://github.com/python/mypy) from 0.961 to 0.991.
- [Release notes](https://github.com/python/mypy/releases)
- [Commits](python/mypy@v0.961...v0.991)

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

Signed-off-by: dependabot[bot] <support@github.com>

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Pete <epwalsh10@gmail.com>

dev-requirements.txt

@@ -2,7 +2,7 @@
 flake8
 
 # Static type checking
-mypy==0.961
+mypy==0.991
 types-requests
 types-cachetools
 types-PyYAML