Drop support for Python 2

Fixes #1551

This PR drops support for Python 2 entirely from the code base.

Things to do in follow-up PRs (should be done after 3.0 is released):

Remove rez.backports.importlib, rez.backports.ordereddict and rez.backports.zipfile.
See if we could unvendor rez.backports.lru_cache. It seems like we slightly modified it?
Remove rez.utils.py23. I did most of the work but we still need to remove get_function_arg_names and load_module_from_file.
See if we can replace rez.utils.data_utils.cached_property with functools.cached_property.
Remove/replace all deprecated (python 2) magic methods (__<method>__).
Remove from __future__ import print_function from the code base.
Look at PRs/commits that added support for Python 3 and see if we missed anything.
Potentially run https://github.com/asottile/pyupgrade.

AcademySoftwareFoundation / rez