MAINT: Bump mypy from 0.910 to 0.991

Bumps [mypy](https://github.com/python/mypy) from 0.910 to 0.991.
- [Release notes](https://github.com/python/mypy/releases)
- [Commits](python/mypy@v0.910...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]

test_requirements.txt

@@ -10,4 +10,4 @@ pickle5; python_version == '3.7' and platform_python_implementation != 'PyPy'
 cffi
 # For testing types. Notes on the restrictions:
 # - Mypy relies on C API features not present in PyPy
-mypy==0.910; platform_python_implementation != "PyPy"
+mypy==0.991; platform_python_implementation != "PyPy"