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/matplotlib.bzl b/debian/matplotlib.bzl
index a2c6683..be520f3 100644
--- a/debian/matplotlib.bzl
+++ b/debian/matplotlib.bzl
@@ -238,7 +238,7 @@
       ],
       data = _data_files + _builtin_so_copied + _system_so_copied + [
           ":usr/share/matplotlib/mpl-data/matplotlibrc",
-      ],
+      ] + native.glob(["etc/**"]),
       imports = ["usr/lib/python" + version + "/dist-packages", version, "."],
       restricted_to = ["@//tools:k8"],
       visibility = ["//visibility:public"],