Pin flake8 to a version that's known good against the 2.16 codebase

setup.py

@@ -55,7 +55,7 @@ testing_extras = [
 
     # For coverage and PEP8 linting
     'coverage>=3.7.0',
-    'flake8>=3.6.0',
+    'flake8==4.0.1',
     'isort==5.6.4',  # leave this pinned - it tends to change rules between patch releases
     'flake8-blind-except==0.1.1',
     'flake8-print==2.0.2',