Fix run-mpl-test-suite for mpl master.

Author: anntzer

run-mpl-test-suite.py

@@ -68,7 +68,7 @@ def _raise_on_image_difference(expected, actual, tol):
 
     with warnings.catch_warnings():  # mpl 3.0
         warnings.filterwarnings("ignore")
-        mpl.use("agg", warn=False, force=True)
+        mpl.use("agg", force=True)
     from matplotlib import pyplot as plt
 
     __orig_switch_backend = plt.switch_backend