chore: block spanner release

Author: daniel-sanche

.librarian/config.yaml

@@ -37,3 +37,7 @@ libraries:
 # Allow release for google-crc32c once this bug is fixed.
   - id: "google-crc32c"
     release_blocked: true
+# TODO(https://github.com/googleapis/google-cloud-python/issues/16600):
+# Allow release for google-cloud-spanner after tests are fixed.
+  - id: "google-cloud-spanner"
+    release_blocked: true