bpo-32911: Add the historical note about the magic number. (GH-7273)

Lib/importlib/_bootstrap_external.py

@@ -246,6 +246,8 @@ _code_type = type(_write_atomic.__code__)
 #     Python 3.7a2  3391 (update GET_AITER #31709)
 #     Python 3.7a4  3392 (PEP 552: Deterministic pycs #31650)
 #     Python 3.7b1  3393 (remove STORE_ANNOTATION opcode #32550)
+#     Python 3.7b5  3394 (restored docstring as the firts stmt in the body;
+#                         this might affected the first line number #32911)
 #     Python 3.8a1  3400 (move frame block handling to compiler #17611)
 #     Python 3.8a1  3401 (add END_ASYNC_FOR #33041)
 #