fix for #3669 Font issue without PyCXX

Looks like CharacterComplement doesn't return a valid unicode string so make it a bytestring in python3

This happens to me for the standard matplotlib font matplotlib/mpl-data/fonts/ttf/Vera.ttf 
not sure why it doesn't happen on travis. 

In any case 'characterComplement' is '\xff\xff\xff\xff6\xff\xff\xfe628R00' for this font on both python2 and python3 which looks rather strange to me. I expected this to be a string of all chars that this font contains so perhaps there is a deeper issue?