fix lint and test coverage

Author: chelsea-lin

db_dtypes/__init__.py

@@ -18,7 +18,6 @@
 import datetime
 import re
 from typing import Optional, Union
-import warnings
 
 import numpy
 import pandas
@@ -31,8 +30,6 @@
 from db_dtypes.json import JSONArray, JSONDtype
 from db_dtypes.version import __version__
 
-from . import _versions_helpers
-
 date_dtype_name = "dbdate"
 time_dtype_name = "dbtime"
 _EPOCH = datetime.datetime(1970, 1, 1)