fix type comparison

biopandas/mmtf/pandas_mmtf.py

@@ -2,21 +2,21 @@
 
 from __future__ import annotations
 
+import os
 import copy
 import gzip
-import os
 import warnings
 from string import ascii_uppercase
 from typing import Any, Dict, List, Union
 from warnings import warn
 
 import numpy as np
 import pandas as pd
-from biopandas.constants import protein_letters_3to1_extended
 from looseversion import LooseVersion
-
 from mmtf import MMTFDecoder, MMTFEncoder, fetch, parse, parse_gzip
 
+from biopandas.constants import protein_letters_3to1_extended
+
 from ..pdb.engines import amino3to1dict, pdb_df_columns, pdb_records
 
 pd_version = LooseVersion(pd.__version__)