[prefix][plotters][user][scatter] Fix IO type used to save plot

matplotlib accepts a StringIO for Python 2 but a wants a BytesIO
with Python 3

beat/core/test/prefix/plotters/user/scatter/1.py

@@ -102,7 +102,10 @@ class Plotter:
         if any(self.legend): ax.legend()
 
         # Returns the image
-        sio = six.StringIO()
+        if six.PY2:
+            sio = six.StringIO()
+        else:
+            sio = six.BytesIO()
 
         if self.mimetype == 'image/png':
             pyplot.savefig(sio, format='png')