Fix trivial differences in wcs/src/docstrings.c between Python versions

I've been noticing that every time I did a build with a different Python version I was still having to rebuild the wcs module every time I switched between versions, even if there were no code changes.  The reason was due to subtle string escape differences--Python 3 was writing '\0' while Python 2 was writing '\x00'.  Likewise Python 2 was escaping ' as \' while Python 3 was not escaping it.