Use correct nth_request in pool.py

nox -s blacken to format

Author: odeke-em

google/cloud/spanner_v1/_helpers.py

@@ -591,9 +591,7 @@ def _check_unavailable(exc):
     resumable_error = (
         any(
             resumable_message in exc.message
-            for resumable_message in (
-                "Service unavailable",
-            )
+            for resumable_message in ("Service unavailable",)
         ),
     )
     if not resumable_error:

google/cloud/spanner_v1/pool.py

@@ -589,7 +589,7 @@ def wrapped_method(*args, **kwargs):
                     return api.batch_create_sessions(
                         request=request,
                         metadata=database.metadata_with_request_id(
-                            database._next_nth_request, attempt.increment(), metadata
+                            nth_request, attempt.increment(), metadata
                         ),
                     )