Bump mypy from 0.931 to 0.991

Bumps [mypy](https://github.com/python/mypy) from 0.931 to 0.991.
- [Release notes](https://github.com/python/mypy/releases)
- [Commits](python/mypy@v0.931...v0.991)

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

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

Author: dependabot[bot]

requirements-test.txt

@@ -2,7 +2,7 @@ black==22.3.0
 flake8==4.0.*
 freezegun==1.1.*
 isort==5.10.*
-mypy==0.931
+mypy==0.991
 pytest==6.2.*
 pytest-cov==3.0.*
 pytest-vcr==1.0.*