This commit removes Six as a dependency for Pelican, replacing the
relevant aliases with the proper Python 3 imports. It also removes
references to Python 2 logic that did not require Six.
* speed up via reduced slugify calls (only call when needed)
* fix __repr__ to not contain str, should call repr on name
* add test_urlwrappers and move URLWrappers tests there
* add new equality test
* cleanup header
additionally:
* Content is now decorated with python_2_unicode_compatible
instead of treating __str__ differently
* better formatting for test_article_metadata_key_lowercase
to actually output the conflict instead of a non descriptive
error
