Feature/extension to python3

I replaced methods with their python 3-variant using the help of the internet. One of the old methods PyString_FromString is returning a new reference, while the its replacement PyUnicode_FromString isn't. Somehow with more knowledge on this topic should check if I fixed it correctly.

The dunepythontest is passing, but valgrind gave me all kind of memory errors for methods I didn't touch. I am not sure what to do with that.