BOM-1013

[awais786]

https://openedx.atlassian.net/browse/BOM-1013 Add tox environments for Django 2.0, 2.1, and 2.2 under Python 3.5

Description:

Describe in a couple of sentence what this PR adds

JIRA:

https://openedx.atlassian.net/browse/BOM-1013

Dependencies:

List dependencies on other outstanding PRs, issues, etc.

Merge deadline:

List merge deadline (if any)

Installation instructions:

List any non-trivial installation instructions.

Testing instructions:

1. Open page A
2. Do thing B
3. Expect C to happen
4. If D happened instead - check failed.

Reviewers:

• [ ] @edx/arch-review
• [ ] tag reviewer

Merge checklist:

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

Author concerns:

List any concerns about this PR - inelegant solutions, hacks, quick-and-dirty implementations, concerns about migrations, etc.

[awais786]

@jmbowman plz review.

[awais786]

Starting from version 3.1, pytest now automatically catches warnings during test execution and displays them at the end of the session. We are using pytest==4.6.6

[awais786]

@jmbowman PR is ready for final review.

[awais786]

@jmbowman I don't have merging rights.

[jmbowman]

Huh, there's an unusual restriction on who can merge to master in this repo. Just merged and tagged.