Update mypy from 0.660 to 0.670

Author: pyup-bot

dev-requirements.txt

@@ -8,7 +8,7 @@ flake8-commas==2.0.0  # Require silicon valley commas
 flake8-quotes==1.0.0  # Require single quotes
 flake8==3.7.5  # Lint
 isort==4.3.4  # Lint imports
-mypy==0.660  # Type checking
+mypy==0.670  # Type checking
 mypy-mypyc==0.660  # Type checking
 pip_check_reqs==2.0.3
 pydocstyle==3.0.0  # Lint docstrings