Assorted edx-django-utils clean-up.

[robrap]

Description:

• Docs and README clean-up.
• Python3 support completed.
• Address review comments from edx-django-utils/pull/2.
• Add latest cookiecutter updates.

JIRA:

ARCH-220

Merge checklist:

• [x] All reviewers approved
• [x] CI build is green
• [x] Version bumped
• [x] Changelog record added
• [x] Documentation updated (not only docstrings)
• [x] Commits are squashed
• [x] PR author is listed in AUTHORS

Post merge:

• [ ] Create a tag
• [ ] Check new version is pushed to PyPi after tag-triggered build is finished.
• [ ] Delete working branch (if not needed anymore)

[robrap]

@ormsbee: I indicated all the file updates that were just coming from the cookiecutter so you could skip those. If you need me to pull those out, let me know. I can also delete those comments if they are a distraction. Thanks.

Also includes fixes for your comments in https://github.com/edx/edx-django-utils/pull/2

[robrap]

@ormsbee: Thanks for the cleaner Python. Please add thumb when you are ready and I'll squash. Thanks.

[robrap]

You beat me by 17 seconds.