skip spanner assignment too

Author: jrmccluskey

sdks/python/apache_beam/ml/rag/ingestion/spanner_it_test.py

@@ -36,7 +36,7 @@
 try:
   from google.cloud import spanner
 except ImportError:
-  spanner = None
+  spanner = None  # type: ignore[assignment]
 
 try:
   from testcontainers.core.container import DockerContainer