Don't use PyInt_FromSsize_t

Change the Python layer to avoid PyInt_FromSsize_t, and remove the
compatibility define.

gdb/ChangeLog
2020-09-15  Tom Tromey  <tromey@adacore.com>

	* python/python-internal.h (PyInt_FromSsize_t): Remove define.
	* python/py-record.c (recpy_element_number): Use
	gdb_py_object_from_longest.
	(recpy_gap_number): Likewise.