Remove F821 from the flake8 ignorelist (#13553)

This is a useful check, and it doesn't seem to generate false positives anymore

Author: AlexWaygood

Diff for: setup.cfg

@@ -38,9 +38,8 @@ exclude =
 #   B007: Loop control variable not used within the loop body.
 #   B011: Don't use assert False
 #   B023: Function definition does not bind loop variable
-#   F821: Name not defined (generates false positives with error codes)
 #   E741: Ambiguous variable name
-extend-ignore = E203,E501,W601,E402,B006,B007,B011,B023,F821,E741
+extend-ignore = E203,E501,W601,E402,B006,B007,B011,B023,E741
 
 [coverage:run]
 branch = true