fix assert in Bind to allow None for the handler.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxPython/Phoenix/trunk@74462 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

etg/event.py

@@ -311,7 +311,7 @@ def run():
             """,
         body="""\
             assert isinstance(event, wx.PyEventBinder)
-            assert callable(handler)
+            assert callable(handler) or handler is None
             assert source is None or hasattr(source, 'GetId')
             if source is not None:
                 id  = source.GetId()