Silence Python 3.8 PY_SSIZE_T_CLEAN warning

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
https://bugs.python.org/issue36381).

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.
1 job for py_ssize_t_clean in 2 minutes and 39 seconds (queued for 34 seconds)
Status Job ID Name Coverage
  Docs
passed #654782
docs

00:02:39