Update mypy from 0.610 to 0.620

Author: pyup-bot

dev-requirements.txt

@@ -9,7 +9,7 @@ flake8-quotes==1.0.0  # Require single quotes
 freezegun==0.3.10  # Freeze time in tests
 hypothesis==3.66.1  # Generate test cases
 isort==4.3.4  # Lint imports
-mypy==0.610  # Type checking
+mypy==0.620  # Type checking
 pip_check_reqs==2.0.3
 pydocstyle==2.1.1  # Lint docstrings
 pyenchant==2.0.0  # Bindings for a spellchecking sytem