pin pyflakes to allow flake8 to succeed

* flake8 monkeypatches pyflakes to add codes to linting results. It
would seem that pyflakes 1.1.0 adds more checks but flake8 doesn't
monkeypatch to expose them. I've submitted a PR to add error code 405 to
be able to express that we ignore it in our setup.cfg
 https://gitlab.com/pycqa/flake8/merge_requests/56

requirements/requirements_dev.txt

@@ -3,6 +3,7 @@ django-debug-toolbar==1.4
 unittest2
 pep8
 flake8
+pyflakes==1.0.0 # Pinned until PR merges https://gitlab.com/pycqa/flake8/merge_requests/56
 pytest
 pytest-cov
 pytest-django