make wx backends compatible with wxPython-Phoenix

This is based on the comments made to my previous PR for this.

Note that wxPython Phoenix snapshots can now be installed with pip:
pip install -U --pre -f http://wxpython.org/Phoenix/snapshot-builds/ wxPython_Phoenix 
I am confused, I think the line should be

```
sorted_filetypes = sort(list(six.iteritems(filetypes)))
```