IECoreBinding : Use Py_ssize_t

MSVC has `SSIZE_T`, but not `ssize_t`, which is not part of the standard
it seems. Python's `pyport.h` file does a `typedef` for it, but
something must have changed between Python `3.7` and `3.10` that causes
that to not be included from what we're using. It does seem that the
official recommendation is to use `Py_ssize_t` now.

Author: ericmehl

include/IECorePython/IECoreBinding.h

@@ -79,9 +79,9 @@ IECOREPYTHON_API std::string repr( T &x );
 
 /// For backwards compatibility with older versions of boost,
 /// which don't provide boost::python::len
-IECOREPYTHON_API inline ssize_t len( const boost::python::object &obj )
+IECOREPYTHON_API inline Py_ssize_t len( const boost::python::object &obj )
 {
-	ssize_t result = PyObject_Length( obj.ptr() );
+	Py_ssize_t result = PyObject_Length( obj.ptr() );
 
 	if( PyErr_Occurred() )
 	{