Bump mypy from 0.790 to 0.931

Bumps [mypy](https://github.com/python/mypy) from 0.790 to 0.931.
- [Release notes](https://github.com/python/mypy/releases)
- [Commits](python/mypy@v0.790...v0.931)

---
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

@@ -3,4 +3,4 @@ pytest-cov==2.11.*
 black==20.8b1
 isort==5.7.*
 flake8==3.8.*
-mypy==0.790
+mypy==0.931