Update isort from 4.3.20 to 4.3.21

Author: pyup-bot

dev-requirements.txt

@@ -7,7 +7,7 @@ dulwich==0.19.11
 flake8-commas==2.0.0  # Require silicon valley commas
 flake8-quotes==2.0.1  # Require single quotes
 flake8==3.7.7  # Lint
-isort==4.3.20  # Lint imports
+isort==4.3.21  # Lint imports
 mypy==0.710  # Type checking
 pip_check_reqs==2.0.3
 pydocstyle==3.0.0  # Lint docstrings