Bump mypy from 0.990 to 0.991

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:development
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

poetry.lock

@@ -21,7 +21,7 @@ python = ">=3.7"
 
 [tool.poetry.dev-dependencies]
 asserts = ">=0.8.0,<0.13"
-mypy = "0.990"
+mypy = "0.991"
 
 [tool.black]
 line-length = 79