Fixing C code changes to be Python 3 compliant

Python 3 introduced many changes in the C API, for example there is no
such thing as PyString, only PyBytes or PyUnicode. Code still does not
run correctly, it looks to be due to the init_spice method generated in
mkwrapper.py. I included my first naive attempts (changing as little as
possible). I will need to explore this further.