RF: remove unused StringIO import
RF: remove unused StringIO import

Helps with Python 3 compatibility as well.

JohnGriffiths/dipy,JohnGriffiths/dipy,samuelstjean/dipy,demianw/dipy,rfdougherty/dipy,beni55/dipy,sinkpoint/dipy,maurozucchelli/dipy,maurozucchelli/dipy,villalonreina/dipy,jyeatman/dipy,FrancoisRheaultUS/dipy,nilgoyyou/dipy,samuelstjean/dipy,mdesco/dipy,jyeatman/dipy,villalonreina/dipy,StongeEtienne/dipy,sinkpoint/dipy,maurozucchelli/dipy,Messaoud-Boudjada/dipy,oesteban/dipy,matthieudumont/dipy,nilgoyyou/dipy,Messaoud-Boudjada/dipy,mdesco/dipy,rfdougherty/dipy,samuelstjean/dipy,matthieudumont/dipy,oesteban/dipy,FrancoisRheaultUS/dipy,demianw/dipy,StongeEtienne/dipy,beni55/dipy