Improve python matplotlib/gtk sandboxing

It was looking for font configs outside of the sandbox. This was
generating a lot of warnings on buster.

Change-Id: I7c2e9c03cc8fbdca7b951e7be404fb20c3a7d3d3
diff --git a/debian/python_gtk.BUILD b/debian/python_gtk.BUILD
index d65101a..e1cdc99 100644
--- a/debian/python_gtk.BUILD
+++ b/debian/python_gtk.BUILD
@@ -63,6 +63,8 @@
 
 _data_files = glob([
     "usr/lib/x86_64-linux-gnu/girepository-1.0/**/*",
+    "usr/share/font*/**",
+    "etc/**",
 ])
 
 _src_copied = ["/".join(f.split("/")[4:]) for f in _src_files]