`wx.lib` on Phoenix: modify `treemixin.py` to allow CustomTreeCtrl support since the switch to the `agwStyle` keywork argument.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxPython/Phoenix/trunk@75574 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775