flake8 updates and cleanups (#3940)

* flake8 updates and cleanups

* Update to flake8 3.7.9.
* Update to flake8-bugbear 20.1.4.
* Only ignore errors in stub files.
* Remove obsolete error counts.
* Sort error codes alphabetically.
* Don't ignore the following errors (unneeded):
    * E704 Multiple statements on one line (def)
    * W504 Line break occurred after a binary operator
    * B303 __metaclass__ used

* Remove obsolete comment

* Ignore F822 undefined name in __all__

.flake8

@@ -1,22 +1,22 @@
 # Some PEP8 deviations are considered irrelevant to stub files:
-# (error counts as of 2017-05-22)
-# 17952 E704 multiple statements on one line (def)
-# 12197 E301 expected 1 blank line
-#  7155 E302 expected 2 blank lines
-#  1463 F401 imported but unused
-#   967 E701 multiple statements on one line (colon)
-#   457 F811 redefinition (should be fixed in pyflakes 2.1.2)
-#   390 E305 expected 2 blank lines
-#     4 E741 ambiguous variable name
+#       E301 expected 1 blank line
+#       E302 expected 2 blank lines
+#       E305 expected 2 blank lines
+#       E701 multiple statements on one line (colon)
+#       E741 ambiguous variable name
+#       F401 imported but unused
+#       F403 import *' used; unable to detect undefined names
+#       F405 defined from star imports
+#       F811 redefinition (should be fixed in pyflakes 2.1.2)
+#       F822 undefined name in __all__
 
 # Nice-to-haves ignored for now
-#  2307 E501 line too long
-
-# Other ignored warnings
-#       W504 line break after binary operator
+#       E501 line too long
 
 [flake8]
-ignore = F401, F403, F405, F811, E301, E302, E305, E501, E701, E704, E741, B303, W504
+per-file-ignores =
+  *.pyi: E301, E302, E305, E501, E701, E741, F401, F403, F405, F811, F822
+
 # We are checking with Python 3 but many of the stubs are Python 2 stubs.
 builtins = StandardError,apply,basestring,buffer,cmp,coerce,execfile,file,intern,long,raw_input,reduce,reload,unichr,unicode,xrange
 exclude = .venv*,@*,.git