SF bug #1242657: list(obj) can swallow KeyboardInterrupt

Fix over-aggressive PyErr_Clear().  The same code fragment appears in
various guises in list.extend(), map(), filter(), zip(), and internally
in PySequence_Tuple().

Objects/abstract.c

@@ -1401,6 +1401,11 @@ PySequence_Tuple(PyObject *v)
 	/* Guess result size and allocate space. */
 	n = PyObject_Size(v);
 	if (n < 0) {
+		if (!PyErr_ExceptionMatches(PyExc_TypeError)  &&
+		    !PyErr_ExceptionMatches(PyExc_AttributeError)) {
+			Py_DECREF(it);
+			return NULL;
+		}
 		PyErr_Clear();
 		n = 10;  /* arbitrary */
 	}