Fix some python2-isms in prep for python3

These ones should work in both versions so we don't have to do them as
part of a global change over.

Change-Id: I2177b329d385c50d706a50a750b6c115ea3ea7ae
diff --git a/frc971/control_loops/python/linear_system.py b/frc971/control_loops/python/linear_system.py
index 105093b..a2e37f0 100755
--- a/frc971/control_loops/python/linear_system.py
+++ b/frc971/control_loops/python/linear_system.py
@@ -215,7 +215,7 @@
 
     U_last = numpy.matrix(numpy.zeros((1, 1)))
     iterations = int(duration / plant.dt)
-    for i in xrange(iterations):
+    for i in range(iterations):
         t = i * plant.dt
         observer.Y = plant.Y
         observer.CorrectObserver(U_last)