⬆️ Update mypy requirement from ^0.971 to ^0.991

Updates the requirements on [mypy](https://github.com/python/mypy) to permit the latest version.
- [Release notes](https://github.com/python/mypy/releases)
- [Commits](python/mypy@v0.971...v0.991)

---
updated-dependencies:
- dependency-name: mypy
  dependency-type: direct:development
...

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

pyproject.toml

@@ -14,7 +14,7 @@ pytest = "^7.0.1"
 black = "^20.8b1"
 isort = "^5.8.0"
 autoflake = "^1.3.1"
-mypy = "^0.971"
+mypy = "^0.991"
 
 [build-system]
 requires = ["poetry>=0.12"]