Use stringIO from six

Python3 fix

Author: jenshnielsen

examples/api/skewt.py

@@ -146,7 +146,7 @@ def _set_lim_and_transforms(self):
     # Now make a simple example using the custom projection.
     from matplotlib.ticker import ScalarFormatter, MultipleLocator
     import matplotlib.pyplot as plt
-    from StringIO import StringIO
+    from six import StringIO
     import numpy as np
 
     # Some examples data