PYTHON-3679 Remove unused import

Author: ShaneHarvey

bson/decimal128.py

@@ -19,7 +19,7 @@
 
 import decimal
 import struct
-from typing import Any, Sequence, Tuple, Type, Union, cast
+from typing import Any, Sequence, Tuple, Type, Union
 
 _PACK_64 = struct.Struct("<Q").pack
 _UNPACK_64 = struct.Struct("<Q").unpack