Prevent boost python registration warning
Prevent boost python registration warning

Now check whether the converter is already registered in a more robust
way.

artas360/pythran,hainm/pythran,artas360/pythran,hainm/pythran,hainm/pythran,pbrunet/pythran,artas360/pythran,pombredanne/pythran,serge-sans-paille/pythran,serge-sans-paille/pythran,pbrunet/pythran,pombredanne/pythran,pombredanne/pythran,pbrunet/pythran