Python 3 Support (WIP)

WORK IN PROGRESS!

If https://github.com/divio/djangocms-text-ckeditor/pull/29 is merged this branch should currently at least start running the tests on 3.3. Still hundreds of errors/failures, but it's a start.

Note:

Even once this pull request is stable, for real support we still need to wait for a bunch of dependencies to make releases, see #1632