Update mypy requirement from ^0.961 to ^0.971 (#45)

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@v0.961...v0.971)

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

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pyproject.toml

@@ -23,7 +23,7 @@ pytest-cov = "^3.0.0"
 [tool.poetry.dev-dependencies]
 black = "^22.1"
 isort = "^5.9.3"
-mypy = "^0.961"
+mypy = "^0.971"
 flake8 = "^4.0.1"
 pytest = "^7.0.0"
 nox = "^2022.1.7"