Use ``extension`` rather than the deprecated ``_extension``.

src/ZODB/BaseStorage.py

@@ -191,7 +191,7 @@ class BaseStorage(UndoLogCompatible):
 
             user = transaction.user
             desc = transaction.description
-            ext = transaction._extension
+            ext = transaction.extension
             if ext:
                 ext = dumps(ext, _protocol)
             else:

src/ZODB/MappingStorage.py

@@ -340,7 +340,7 @@ class TransactionRecord:
         self.tid = tid
         self.user = transaction.user
         self.description = transaction.description
-        extension = transaction._extension
+        extension = transaction.extension
         self.extension = extension
         self.data = data