py: Fix segfault in namedtuple when name is a non-interned string

- namedtuple was wrongly using MP_OBJ_QSTR_VALUE instead of mp_obj_str_get_qstr,
so when passed a non-interned string it would segfault; fix this by using mp_obj_str_get_qstr
- store the namedtuple field names as qstrs so it is not needed to use mp_obj_str_get_qstr
everytime the field name has to be accessed. This also slighty increases performance when
fetching attributes