build(deps-dev): update mypy requirement from ^0.920 to ^0.921 (#157)

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@v0.920...v0.921)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

pyproject.toml

@@ -28,7 +28,7 @@ flake8-bugbear = "^21.4.3"
 flake8-isort = "^4.0.0"
 flake8-pyi = "^20.10.0"
 isort = "^5.9.2"
-mypy = "^0.920"
+mypy = "^0.921"
 pytest = "^6.2.4"
 pytest-cov = "^3.0.0"
 pytest-mock = "^3.6.1"