Reject pycodestyle 2.1.0

Fix for PyCQA/pycodestyle#400 created some false positives for E305
The pull-request in PyCQA/pycodestyle#593 fixing this issue has been
done after 2.1.0.

Author: enzbang

tox.ini

@@ -28,7 +28,7 @@ commands=
 [testenv:checkstyle]
 basepython = python
 deps =
-      pycodestyle>=2.0.0
+      pycodestyle>=2.0.0,!=2.1.0
       pyflakes>=1.2.3
       pydocstyle>=1.0.0
 commands =