code: def append_form(
              self,
              obj: Union[Sequence[Tuple[str, str]],
                         Mapping[str, str]],
              headers: Optional['MultiMapping[str]']=None
      ) -> Payload:
          
          assert isinstance(obj, (Sequence, Mapping))
  
          if headers is None:
              headers = CIMultiDict()
  
          if isinstance(obj, Mapping):
              obj = list(obj.items())
          data = urlencode(obj, doseq=True)
  
          return self.append_payload(
              StringPayload(data, headers=headers,
                            content_type='application/x-www-form-urlencoded'))
docs: Helper to append form urlencoded part.