Bump mypy from 0.990 to 0.991 (#455)

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
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

mypy_requirements.txt

@@ -1,2 +1,2 @@
 # Requirements to run mypy itself. Mypy executable exists in a separate venv.
-mypy==0.990
+mypy==0.991