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 
On 12/5/2014 16:43, Benjamin Root wrote:
>
> In examples/user_interfaces/embedding_in_wx3.py:
>
> > @@ -23,7 +23,10 @@
> >
> >  # Used to guarantee to use at least Wx2.8
> >  import wxversion
> > -wxversion.ensureMinimal('2.8')
> > +#wxversion.ensureMinimal('2.8')
> > +#wxversion.select('2.8')
> > +#wxversion.select('2.9.5') # 2.9.x classic
> > +wxversion.select('3.0.1-msw-phoenix', optionsRequired=True) # 3.0 phoenix
>
> let's come up with a consistent import approach in all examples. 
> Having different imports without explanation is confusing.
>
My suggestion would be:

# matplotlib requires wxPython 2.8+
# set the wxPython version in lib\site-packages\wx.pth file
# or if you have wxversion installed un-comment the lines below
#import wxversion
#wxversion.ensureMinimal('2.8')

Werner