fix assert in Bind to allow None for the handler.

git-svn-id: http://svn.wxwidgets.org/svn/wx/wxPython@74462 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Merge of PR 11 with the whitespace-only changes omitted.

See https://github.com/RobinD42/Phoenix/pull/11

Mostly phoenix-port changes.

git-svn-id: http://svn.wxwidgets.org/svn/wx/wxPython@74463 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775