fix missing __loader__ for bootstraped builtin modules

Author: cosminbasca

graalpython/lib-graalpython/final_patches.py

@@ -1,4 +1,4 @@
-# Copyright (c) 2019, Oracle and/or its affiliates. All rights reserved.
+# Copyright (c) 2019, 2020, Oracle and/or its affiliates. All rights reserved.
 # DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 #
 # The Universal Permissive License (UPL), Version 1.0
@@ -42,8 +42,10 @@
 
 _imp.cache_all_file_modules()
 
-from importlib._bootstrap import _spec_from_module
+from importlib._bootstrap import BuiltinImporter
 for name in ['importlib._bootstrap', 'importlib._bootstrap_external', '_descriptor']:
     mod = sys.modules.get(name, None)
     if mod:
-        setattr(mod, '__spec__', _spec_from_module(mod))
+        spec = BuiltinImporter.find_spec(name)
+        setattr(mod, '__spec__', spec)
+        setattr(mod, '__loader__', spec.loader)