Update cython.m4 and pythonhead.m4 files to be compatible with py2/3 pythonhead.m4 changes taken from: https://raw.githubusercontent.com/posborne/dbus-python/master/m4/am-check-python-headers.m4 cython.m4 changes taken from: https://raw.githubusercontent.com/alisw/rivet/master/m4/cython.m4