Bump mypy from 0.971 to 0.991

Bumps [mypy](https://github.com/python/mypy) from 0.971 to 0.991.
- [Release notes](https://github.com/python/mypy/releases)
- [Commits](python/mypy@v0.971...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

@@ -90,7 +90,7 @@ sphinx = "^4.0.2"
 sphinx-rtd-theme = "^0.5.2"
 strict-rfc3339 = "^0.7"
 webob = "*"
-mypy = "^0.971"
+mypy = "^0.991"
 starlette = "^0.23.1"
 httpx = "^0.23.3"