Fix inside_transaction issue

Author: JoudZouzou

google/cloud/sqlalchemy_spanner/sqlalchemy_spanner.py

@@ -48,7 +48,7 @@
 @listens_for(Pool, "reset")
 def reset_connection(dbapi_conn, connection_record):
     """An event of returning a connection back to a pool."""
-    if dbapi_conn.connection.inside_transaction:
+    if getattr(dbapi_conn.connection, "inside_transaction", False):
         dbapi_conn.connection.rollback()
 
     if getattr(dbapi_conn.connection, "staleness", None) is not None: