Cleaned up a few more things about the testing configuration now that we've deployed Django 2.2 to production:

* Go back to a single version of django-oauth-toolkit, tox doesn't need to vary it anymore
* Go back to PyPI release of django-method-override
* Remove no longer needed dependency on python-oauth2 (we already removed the package it was used by)
* Update prepare_xdist_nodes.sh with new Django requirements file options

This reverts commit 27480a25.

* Updating Python Requirements

* Constrain django-cookies-samesite until dep bug fixed

* Fix PEP 8 problems revealed by pycodestyle upgrade

Co-authored-by: Tim McCormack <tmccormack@edx.org>
Co-authored-by: Jeremy Bowman <jbowman@edx.org>

Improved authentication metrics.

This adds a graded icon for LTI units. This was meant to go into
a recent commit, but I only updated github.in not the txt requirement
files.

AA-75

This adds a missing migration file to change the plural name of a
model.

These were pinned because they drop support for Django<2.2 (or were
planning on it, in the latter case.)

No related changes in computed requirements.

ARCHBOM-1105

TNL-7184

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