Set backend in swerve matplotlib users

On some systems the default backend doesn't work. Replicate the setup
from build_tests/matplotlib_example.py.

Change-Id: I28cd8e4d3fc442a9aea0dcc26490ec4b5ca7ed83
Signed-off-by: James Kuszmaul <jabukuszmaul+collab@gmail.com>
diff --git a/frc971/control_loops/swerve/simulation.py b/frc971/control_loops/swerve/simulation.py
index 4eed97b..d7e2fec 100644
--- a/frc971/control_loops/swerve/simulation.py
+++ b/frc971/control_loops/swerve/simulation.py
@@ -6,6 +6,7 @@
 from frc971.control_loops.python import control_loop
 from frc971.control_loops.python import controls
 
+import matplotlib
 from matplotlib import pylab
 import sys
 import gflags
@@ -13,6 +14,8 @@
 
 FLAGS = gflags.FLAGS
 
+matplotlib.use("GTK3Agg")
+
 
 class SwerveSimulation(object):