Upgrade pytest

[toolness]

This upgrades the following packages:

• pytest
• pytest-django
• pytest-cov

To do

• It seems pytest now logs regular python warnings as pytest warnings. We should resolve them. Update: This is done, though the fact that there was a typo in a graphql-core deprecation warning, which has been fixed but is not yet part of an official release, complicated things a bit.

• Fix this warning that is happening in CircleCI but not on my local dev machine:

.venv/lib/python3.7/site-packages/botocore/vendored/requests/packages/urllib3/_collections.py:1
  /root/tenants2/.venv/lib/python3.7/site-packages/botocore/vendored/requests/packages/urllib3/_collections.py:1: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
    from collections import Mapping, MutableMapping