PyHEADTAIL v1.13.4

On the way to more and more python3 support:

cython requires compiler_directives={'language_level' : "3"} for python3 support -- in order to remain compatible with python2, we include

cythonize(extensions, compiler_directives={'language_level' : sys.version_info[0]})

into the setup.py (thanks to stackoverflow).