docs: def create(cls, mp, part_number, stream=None, **kwargs):
          """Create a new part object in a multipart object."""
          if part_number < 0 or part_number > mp.last_part_number:
              raise MultipartInvalidPartNumber()
  
          with db.session.begin_nested():
              obj = cls(
                  multipart=mp,
                  part_number=part_number,
              )
              db.session.add(obj)
          if stream:
              obj.set_contents(stream, **kwargs)
          return obj