Some flake8 fixes.

In regarding to @alex 's tweet, here comes some flake8 fixes for the views module.

In my opinion, many of the E501 (the ones with huge lines) errors just don't worth the effort to fix because they're representing some templates, and there are some F401 errors in /views/generic/__init__.py but I don't think there is a major-refactor-free way to do this.