Refresh Python dependencies

Author: mdmintz

requirements.txt

@@ -116,7 +116,7 @@ pytest-cov==2.12.1;python_version<"3.6"
 pytest-cov==3.0.0;python_version>="3.6"
 flake8==3.7.9;python_version<"3.5"
 flake8==3.9.2;python_version>="3.5" and python_version<"3.6"
-flake8==4.0.1;python_version>="3.5"
+flake8==4.0.1;python_version>="3.6"
 pyflakes==2.1.1;python_version<"3.5"
 pyflakes==2.4.0;python_version>="3.5"
 pycodestyle==2.5.0;python_version<"3.5"