Provide Python 3 matplotlib

Once I get around to making numpy/scipy imported for Python 3, I'll
probably upgrade everything to Python 3 and drop Python 2 support from
the codebase, but I don't feel like doing that quite yet.

Change-Id: I164c8fbb3ce922a83b56e46f5cc432004c3a989c
diff --git a/motors/python/BUILD b/motors/python/BUILD
index d5d74ae..5eafc3a 100644
--- a/motors/python/BUILD
+++ b/motors/python/BUILD
@@ -13,7 +13,7 @@
         "//external:python-gflags",
         "//external:python-glog",
         "//frc971/control_loops/python:controls",
-        "@matplotlib",
+        "@matplotlib_repo//:matplotlib2.7",
     ],
 )