python: Add pypath_converter
    
    In Python 3 we would like to handle paths as either unicode strings
    or bytes. For this purpose we have a c-API called PyUnicode_FSConverter.
    In this patch the missing API is implemented for Py2 based on Py3 [1]
    and a unified no-cleanup behavior is enforced for Py3 path conversion as well.
    
    [1] https://github.com/python/cpython/blob/master/Objects/unicodeobject.c#L3818