fix imports

Author: sycai

bigframes/core/compile/ibis_compiler/scalar_op_registry.py

@@ -16,7 +16,7 @@
 
 import functools
 import typing
-from typing import cast, Union
+from typing import cast
 
 from bigframes_vendored import ibis
 import bigframes_vendored.ibis.expr.api as ibis_api