Bump mypy from 0.770 to 0.780

Bumps [mypy](https://github.com/python/mypy) from 0.770 to 0.780.
- [Release notes](https://github.com/python/mypy/releases)
- [Commits](python/mypy@v0.770...v0.780)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Author: dependabot-preview[bot]

dev-requirements.txt

@@ -16,7 +16,7 @@ flake8-quotes==3.2.0  # Require single quotes
 flake8==3.8.2  # Lint
 freezegun==0.3.15
 isort==4.3.21  # Lint imports
-mypy==0.770  # Type checking
+mypy==0.780  # Type checking
 pip-check-reqs-pip-gte-20==2.0.3.4
 pydocstyle==5.0.2  # Lint docstrings
 pyenchant==3.1.1  # Bindings for a spellchecking sytem