Regression: import lalinspiral.thinca fails on Python 3

A regression was introduced in !580 (merged) that caused import lalinspiral.thinca to raise an error in Python 3:

$ python -c 'import lalinspiral.thinca'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib/python3.7/site-packages/lalinspiral/thinca.py", line 41, in <module>
    from . import _thinca
ImportError: dynamic module does not define module export function (PyInit__thinca)

I suggest using the six.h header file that is inside the gnuscripts directory to work around the differences in the Python 2 and 3 module init API. Examples can be found in the other non-SWIG Python C extensions in lalsuite.