code: def get_pk_colnames(table_: Table) -> List[str]:
      
      pk_names = []  # type: List[str]
      for col in table_.columns:
          if col.primary_key:
              pk_names.append(col.name)
      return pk_names
docs: If a table has a PK, this will return its database column name(s);
      otherwise, ``None``.