This adds RemovedInRBTools50Warning and RemovedInRBTools60Warning,

which will be used for upcoming changes that deprecate some older

functionality.
RemovedInRBTools40Warning will remain in the codebase until all

pending deprecations for 4.0 have been removed.
This also introduces the first usage of Python type hints to our

codebase. This is new for us, so we'll need to develop best practices

over time.

Made use of this in an in-progress change.
Unit tests pass.