BF: fix funky error using __import__ on cvxopt
BF: fix funky error using __import__ on cvxopt

Very strange error importing cvxopt with python3 and __import__; maybe
this is because I was using an half-documented form of the function
(passing an empty string in `fromlist`).

Don't pass empty string when it's not needed in optpkg

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