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/frc971/control_loops/python/BUILD b/frc971/control_loops/python/BUILD
index b6290d4..a4b8fb8 100644
--- a/frc971/control_loops/python/BUILD
+++ b/frc971/control_loops/python/BUILD
@@ -112,6 +112,7 @@
         ":drivetrain",
         ":python_init",
         "//external:python-glog",
+        "@matplotlib_repo//:matplotlib3",
     ],
 )