Remove six from isort standard libraries

isort seems to have switched to `frozenset` for `known_standard_library`, which broke the way we're overriding it here to add `six`. This caused only `six` to be recognized as a standard library, and everything else as third-party. This is an explicit behavior change (see PyCQA/isort#1238).

We no longer use `six` so we can remove this setting

tox.ini

@@ -54,6 +54,6 @@ multi_line_output=3
 skip=.tox,migrations
 use_parentheses=1
 known_django=django
-known_future_library=future,six
+known_future_library=future
 default_section=THIRDPARTY
 sections=FUTURE,STDLIB,DJANGO,THIRDPARTY,FIRSTPARTY,LOCALFOLDER