Remove duplicate constraints in requirements/constraints.txt uncovered by the recent change in pip-compile output format. I sorted the pinned dependencies by name to try and make it more obvious if this happens again. I also upgraded to pip-tools 4.5.1, which removes the line numbers from the enhanced pip-compile output added in 4.5.0, which should reduce future diff churn and merge conflicts but means that there's a large diff this one last time.
Also unpin edx-search again after the previous change to do that was apparently broken by a merge conflict.
Unpin several more outdated dependencies whose changelogs don't contain any significant backwards incompatible changes.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.
The xblock counting script had a requirements file that was not
previously being tracked and had an out of date version of requests.
Add it to the `upgrade` make target so that it stays up to date.
