Update mypy from 0.780 to 0.781 (#379)

Author: pyup-bot

requirements-testing.txt

@@ -4,7 +4,7 @@ flake8==3.8.3
 pytest==5.4.3
 pytest-cov==2.10.0
 autopep8==1.5.3
-mypy==0.780
+mypy==0.781
 py-spy==0.3.3
 codecov==2.1.7
 tox==3.15.2