Remove flake8-putty

This is not compatible with current version of flake8 and doesn't sem to be
needed anymore. This could be removed anyway with githubs super linter
anyway.

setup.cfg

@@ -2,12 +2,18 @@
 universal = 1
 
 [flake8]
-exclude = extras, build, dist, node_modules, migrations, settings.py, docs
 # H101: Use TODO(NAME)
 # W503: line break before binary operator
 ignore = H101,W503
 max-line-length = 100
-putty-ignore =
+exclude =
+    extras,
+    build,
+    dist,
+    node_modules,
+    migrations,
+    settings.py,
+    docs,
     **/urls.py : E501
     **/config/apps.py : F401
     **/core/apps.py : F401