Bump mypy from 0.990 to 0.991 (#628)

Bumps [mypy](https://github.com/python/mypy) from 0.990 to 0.991.
- [Release notes](https://github.com/python/mypy/releases)
- [Commits](python/mypy@v0.990...v0.991)

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

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>

mypy-requirements.txt

@@ -1,4 +1,4 @@
-mypy==0.990
+mypy==0.991
 black>=19.10b0
 types-pkg_resources
 types-requests