[Cython] PyUnicode_Tailmatch

[Josh Ayers]

Cython devs,

In the function __Pyx_PyUnicode_Tailmatch, the return type of
PyUnicode_Tailmatch is assumed to be in int.  See line 543 of
Cython/Utility/StringTools.c.  

PyUnicode_Tailmatch actually returns a Py_ssize_t.  See:
https://docs.python.org/3/c-api/unicode.html#c.PyUnicode_Tailmatch.

For reference, there was previously an error in the Python documentation
of this function.  The documentation mistakenly said it returned an int.
 This was fixed via Python issue 22580:
https://bugs.python.org/issue22580.

Thanks,
Josh Ayers