Bump mypy from 0.920 to 0.921

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:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

mypy-requirements.txt

@@ -1,3 +1,3 @@
-mypy==0.920
+mypy==0.921
 types-requests
 types-setuptools