Fix flake8 config (#1181)

setup.cfg

@@ -5,6 +5,4 @@
 [flake8]
 max-line-length = 80
 # E203: This warning is not PEP 8 compliant.
-# E402: Allow the pythonpath modifications before repo-local imports.
-# W503: flake8 v3.8.4 is inconsistent with black v20.8b1 (pre-commit run -a).
-ignore = E203,E402,W503
+extend-ignore = E203