Use BytesIO from io.

[jenshnielsen]

Fix bad merge from 8fe495a

As noted by @WeatherGod in #4605 master is failing.

That is because the merge of the six branch (#4501) changed it from

from io import BytesIO 
to

from matplotlib.externals.six import BytesIO
six.BytesIO is stringIO.stringIO in python 2

while #4603 changed it to use BytesIO all over.

I.e I think the merge in 8fe495a is wrong

[WeatherGod]

+1 from me when travis clears