* Constraining edx-rest-api-client

* Decreased pin on edx-rest-api-client from <3.0.0 to <2.0.0

Unpin several more outdated dependencies whose changelogs don't contain any significant backwards incompatible changes. Also add "moto" to the list of packages to uninstall from existing environments, since it requires a jsondiff version that clashes with the one we now use (triggering a harmless but distracting warning on dependency updates).

We can potentially stop using path.py/path altogether by switching to pathlib in the Python 3 standard library, but that merits a separate PR of its own.

Also, note that I'm not actually unpinning freezegun; different PRs restricted it in both constraints.txt and test.in, I'm just removing the latter redundant constraint.

Removing the moto.

This is step 3 of DE-1824.

Also update:
* six 1.13.0 => 1.14.0
* edx-django-utils 2.0.2 => 2.0.3
* skip test that checks all model changes have migrations.  (We defer
  this to step 4.)

* Switch make upgrade to Python 3.5

* bok-choy fixes, constrain django-debug-toolbar

* Remove flaky bok-choy test

This commit upgrades the version of pymongo from 2.x to 3.x, removing usages to deprecated functions usage and fixing tests where necessary.

This version of pymongo supports MongoDB 2.x all the way up to 4.2, and this ensures that the platform will be able to run on a supported MongoDB version in the next release.

guincorn just released its version on 9th-nov-2019 (20.0.0)
and it requires Python 3.x >= 3.4.
so adding constraint < 20.0.0

execute make upgrade

Bump the version.

Upgraded xblock-google-drive to latest commit hash.

Some change in the requirements here may be responsible for increased
CPU and memory usage.  This is causing production LMS for edx.org to run
into lots of performance issues.  Reverting for now until we can narrow
it down to which change caused the issue.

Fix for test that broke related to DRF;

added the updated dependency for oauth2

added the commit hash instead of branch

minor fix

changes made by make upgrade