Unpin several more outdated dependencies whose changelogs don't contain any significant backwards incompatible changes.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.
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.
