Make it easier to emit deprecation warnings using our modern classes.
Review Request #1831 — Created Jan. 22, 2020 and updated
This adds a convenience function to `BaseRemovedInReviewBoardVersionWarning` that wraps `warnings.warn()`, passing in the owning class and a default stack level. This massively simplifies the process of emitting a warning, and is one fewer import and two fewer parameters for files. Testing Done: Unit tests pass. Verified the output of warnings manually. Reviewed at https://reviews.reviewboard.org/r/10777/