Issue #19437: Fix convert_op_cmp() of decimal.Decimal rich comparator, handle
PyObject_IsInstance() failure
Issue #19437: Fix dec_format() of the _decimal module, handle dec_strdup()
failure (memory allocation failure): raise a MemoryError exception
Change style to match the surrounding code (no early returns).
Move PyErr_NoMemory() closer to the failure.
Valgrind: suppress false positive in _PyOS_GetOpt (getopt.c:84) (Invalid read
of size 8: wcscmp (wcscmp.S:464))