Fix introspection PK comment more.

        if pk_column:

            # SQLite doesn't actually give a name to the PK constraint,

            # so we invent one. This is fine, as the SQLite backend never

            # deletes PK constraints by name.

            # deletes PK constraints by name, as you can't delete constraints

            # in SQLite; we remake the table with a new PK instead.

            constraints["__primary__"] = {

                "columns": set([pk_column]),

                "primary_key": True,