py/objtype: Check and prevent delete/store on a fixed locals map.

Note that the check for elem!=NULL is removed for the
MP_MAP_LOOKUP_ADD_IF_NOT_FOUND case because mp_map_lookup will always
return non-NULL for such a case.

tests/basics/del_attr.py

@@ -30,3 +30,10 @@ try:
     del c.x
 except AttributeError:
     print("AttributeError")
+
+# try to del an attribute of a built-in class
+try:
+    del int.to_bytes
+except (AttributeError, TypeError):
+    # uPy raises AttributeError, CPython raises TypeError
+    print('AttributeError/TypeError')