Bump mypy from 0.920 to 0.921 (#1189)

Bumps [mypy](https://github.com/python/mypy) from 0.920 to 0.921.
- [Release notes](https://github.com/python/mypy/releases)
- [Commits](python/mypy@v0.920...v0.921)

---
updated-dependencies:
- dependency-name: mypy
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

poetry.lock

@@ -65,7 +65,7 @@ trio = "^0.19"
 attrs = "^21.2"
 httpx = "^0.21"
 
-mypy = "^0.920"
+mypy = "^0.921"
 wemake-python-styleguide = "^0.16"
 flake8-pytest-style = "^1.5"
 flake8-pyi = "^20.10"