Fix compile for Python >= 3.0.

Python docs tells us more about string functions:
"These functions have been renamed to PyBytes_* in Python 3.x. Unless
otherwise noted, the PyBytes functions available in 3.x are aliased to
their PyString_* equivalents to help porting."
(http://docs.python.org/c-api/string.html)

Use #defines to map the new names and fix the compile for Python >= 3.0.

Thanks Olivier Fauchon <redacted> for the patch.