> I want to avoid adding a new warning class if one of the standard library does the job.

In terms of warnings, I understand "does the job" as: where a user might want to silence a particular warning or turn it into an exception,  it should be easy, unambiguous and version-invariant.

This means it should have a specific class to distinguish it from other warnings in the module if that seems appropriate. @ogrisel suggested it might be in this case. Backwards-compatibility would have me extend from `UserWarning`.