Use start and stop methods of patch in scenario

This ensure that the modification on the object are removed at the end of the
scenario.

issue10422
review332891002
(grafted from 25729b538f2df592e6a370aa0c75a6b2ac1a1684)

--HG--
branch : 6.0

tests/scenario_marketing_automation.rst

@@ -13,9 +13,10 @@ Imports::
 
 Patch sendmail_transactional::
 
-    >>> from unittest.mock import MagicMock
+    >>> from unittest.mock import patch
     >>> from trytond.modules.marketing_automation import marketing_automation
-    >>> marketing_automation.sendmail_transactional = smtp_calls = MagicMock()
+    >>> smtp_calls = patch.object(
+    ...     marketing_automation, 'sendmail_transactional').start()
 
 Activate modules::