Some of our warnings code predated the Djblets-specific deprecation

warning classes, and these made it hard for us to search for

deprecations to remove in new releases. Those have been updated to use

RemovedInDjblets30Warning.
Unit tests have been updated to better check for warnings by using

assertWarns instead of catch_warnings. This slims down the code and

makes the behavior more consistent.
This led to finding one assertion check that was no longer valid, and

some calls to deprecated code that didn't check for deprecation warnings

(which leaked warnings to the test runner). Those have been updated to

catch warnings.

Unit tests pass on all supported versions of Python and Django.