add versionchanged for source

Author: Aniketsy

Doc/library/warnings.rst

@@ -638,6 +638,9 @@ Available Context Managers
       - ``source``: the original object that generated the warning (if available), or ``None``
       - ``module``: the module name where the warning occurred (:class:`str`), or ``None``
 
+    .. versionchanged:: 3.6
+      The ``source`` attribute was added.
+
     .. versionchanged:: 3.15
       The ``module`` attribute was added.