In Python 3, `PyArg_ParseTuple` and friends can return either an int
or a Py_ssize_t type for the length in an argument of the form
`"s#"`, `"y#"`, `"u#"`, etc, controlled by whether the preprocessor
macro `PY_SSIZE_T_CLEAN` is defined when the header `Python.h` is
included. A future version of Python will only support `PY_SSIZE_T`.
In Python 3.8, there was a deprecation warning added (see
Define the `PY_SSIZE_T_CLEAN` macro to suppress the hundreds or
thousands of deprecation warnings that may be printed when reading
a LIGO_LW file.
See also lscsoft/glue!69.