Fix matplotlib sandboxing

This appears to fix plotting with matplotlib in python3.  This lets
things like //y2020/control_loops/python:finisher --plot work.

Change-Id: I8ff1c91cf078e3aa8b4d5660881c22c8de66f051
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/debian/matplotlib.BUILD b/debian/matplotlib.BUILD
index 881521c..ad44398 100644
--- a/debian/matplotlib.BUILD
+++ b/debian/matplotlib.BUILD
@@ -2,10 +2,5 @@
 
 build_matplotlib(
     "3",
-    tkinter_py_version = "3.5",
-)
-
-build_matplotlib(
-    "2.7",
-    copy_shared_files = False,
+    tkinter_py_version = "3.7",
 )