objnamedtuple: Accept field list as a string.

This change is required to unbreak some CPython stdlib modules (as included
into micropython-lib).

py/objnamedtuple.c

@@ -30,6 +30,7 @@
 #include "py/nlr.h"
 #include "py/objtuple.h"
 #include "py/runtime.h"
+#include "py/objstr.h"
 
 #if MICROPY_PY_COLLECTIONS
 
@@ -169,6 +170,11 @@ STATIC mp_obj_t new_namedtuple_type(mp_obj_t name_in, mp_obj_t fields_in) {
     qstr name = mp_obj_str_get_qstr(name_in);
     mp_uint_t n_fields;
     mp_obj_t *fields;
+    #if MICROPY_CPYTHON_COMPAT
+    if (MP_OBJ_IS_STR(fields_in)) {
+        fields_in = mp_obj_str_split(1, &fields_in);
+    }
+    #endif
     if (!MP_OBJ_IS_TYPE(fields_in, &mp_type_list)) {
         nlr_raise(mp_obj_new_exception_msg(&mp_type_TypeError, "list required"));
     }