Support "related object" association

[Julian]

Occasionally, one has a situation such as:

• foo, a deprecated callable, which raises _SomeKindOfException

where _SomeKindOfException is raised only by foo, and perhaps isn't itself public.

Once foo is removed, it is potentially easy to forget to remove _SomeKindOfException.

It may be nice to provide a way to ensure _SomeKindOfException is marked as related to foo inside of foo's deprecation. This may be so that e.g. #1 or #14 are able to raise errors if foo gets removed but _SomeKindOfException remains.