bpo-43950: ensure source_line is present when specializing the traceback

[isidentical]

There seems to be different cases where the _PyDisplay_Line() function returns 0 without displaying the actual line but also without having any errors raised.

cpython/Python/traceback.c

Lines 414 to 420 in 4512848

	/* use the right encoding to decode the file as unicode */
	fd = PyObject_AsFileDescriptor(binary);
	if (fd < 0) {
	Py_DECREF(io);
	Py_DECREF(binary);
	return 0;
	}

cpython/Python/traceback.c

Lines 436 to 446 in 4512848

	if (fob == NULL) {
	PyErr_Clear();
	res = _PyObject_CallMethodIdNoArgs(binary, &PyId_close);
	Py_DECREF(binary);
	if (res)
	Py_DECREF(res);
	else
	PyErr_Clear();
	return 0;
	}

This patch ensures that the line is retrieved and displayed.

https://bugs.python.org/issue43950