Fixed linting.

Signed-off-by: Daniel Scheffler <danschef@gfz-potsdam.de>

geoarray/baseclasses.py

@@ -27,7 +27,7 @@ import warnings
 from pkgutil import find_loader
 from collections import OrderedDict
 from copy import copy, deepcopy
-from typing import Union  # noqa F401
+from typing import Union, Optional  # noqa F401
 
 import numpy as np
 from osgeo import gdal, gdal_array, gdalnumeric