docs: def _find_subclass(cls, module):
          """
          Attempt to find subclass of :class:`RelationBase` in the given module.
  
          Note: This means strictly subclasses and not :class:`RelationBase` itself.
          This is to prevent picking up :class:`RelationBase` being imported to be
          used as the base class.
          """
          for attr in dir(module):
              candidate = getattr(module, attr)
              if (isclass(candidate) and issubclass(candidate, cls) and
                      candidate is not RelationBase):
                  return candidate
          return None