This introduces RemovedInDjblets40Warning and

RemovedInDjblets50Warning and sets RemovedInNextDjbletsVersionWarning

to RemovedInDjblets40Warning.
RemovedInDjblets20Warning has been removed, as it's no longer needed.
BaseRemovedInDjbletsVersionWarning has been updated to include a new

warn() method, making it easier to emit deprecation warnings.

Unit tests pass.
Began using RemovedInDjblets40Warning in new code.