chore(deps-dev): Bump mypy from 0.800 to 0.812 (#340)

Bumps [mypy](https://github.com/python/mypy) from 0.800 to 0.812.
- [Release notes](https://github.com/python/mypy/releases)
- [Commits](python/mypy@v0.800...v0.812)

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