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.
- Reimplement `djcelery.common.respect_language` utility function so we
can drop the dependency.
- Loosen `celery` pinning to a range, which brings in a fix for a
possible crash bug
Also, pin `path` to 13.1.0 for now in order to continue supporting py35.
Django-auth-toolkit update
Pinned the latest commit from github to get Django2.2 support in testing.
Pinned the zipp<2.0.0 to get Python<3.5 support too.
