need wxPython reviewers for PR

We have a PR to fix a problem in backend_wx.py, and as far as I know
most of us who have been reviewing PRs recently are not WX users. Hence
this call for assistance: if you use the wxagg backend, and can build
matplotlib from source, please try out the patch and review it:

https://github.com/matplotlib/matplotlib/pull/7652

Thanks.

Eric