math/py-matplotlib: Allow build for Python 3.x

• x11-toolkits/{py-gtk2,py-wxPython28} do not support Python 3.x, so exclude the GTKBACKEND, GTKAGGBACKEND, and WXAGGBACKEND in that case.

PR: 213636
Reported by: lbartoletti@tuxfamily.org
Submitted by: rsmith@xs4all.nl (based on)
Reviewed by: mat
Approved by: maintainer timeout
Differential Revision: https://reviews.freebsd.org/D13377