FIX: Update GTK3Agg backend export name for consistency

Author: greglucas

lib/matplotlib/backends/backend_gtk3agg.py

@@ -70,5 +70,5 @@ def blit(self, bbox=None):
 
 
 @_BackendGTK3.export
-class _BackendGTK3Cairo(_BackendGTK3):
+class _BackendGTK3Agg(_BackendGTK3):
     FigureCanvas = FigureCanvasGTK3Agg