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 
Hhm, but on Py3 that gives me an except:
with this:
        sorted_filetypes = filetypes.items()
        #sorted_filetypes = list(six.iteritems(filetypes))
        sorted_filetypes.sort()

File "D:\devTools\matplotlib-Phoenix\examples\user_interfaces\embedding_in_wx2.py", line 108, in <module>
  app.MainLoop()
File "c:\Python34\Lib\site-packages\wx\core.py", line 1879, in MainLoop
  rv = wx.PyApp.MainLoop(self)
File "c:\Python34\Lib\site-packages\matplotlib\backends\backend_wx.py", line 1696, in save_figure
  filetypes, exts, filter_index = self.canvas._get_imagesave_wildcards()
File "c:\Python34\Lib\site-packages\matplotlib\backends\backend_wx.py", line 902, in _get_imagesave_wildcards
  sorted_filetypes.sort()

builtins.AttributeError: 'dict_items' object has no attribute 'sort'