mail: document behavior of Python 3

test-notify.t (and possibly other tests) are failing on Python 3
because email.message.Message is now aware of encodings and
attempts to roundtrip values with the specified message encoding.

Python 2 doesn't perform this roundtripping. We have tests with
non-ascii data being serialized to a message that claims to use
ascii encoding.

I /think/ Mercurial's behavior may be buggy here. But I'm not
sure.

I'm documenting the behavior so the next person who looks into
this doesn't start from scratch like I did.

Differential Revision: https://phab.mercurial-scm.org/D5714

mercurial/mail.py

@@ -243,6 +243,13 b' def mimetextqp(body, subtype, charset):'
             cs.body_encoding = email.charset.QP
             break
 
+    # On Python 2, this simply assigns a value. Python 3 inspects
+    # body and does different things depending on whether it has
+    # encode() or decode() attributes. We can get the old behavior
+    # if we pass a str and charset is None and we call set_charset().
+    # But we may get into  trouble later due to Python attempting to
+    # encode/decode using the registered charset (or attempting to
+    # use ascii in the absence of a charset).
     msg.set_payload(body, cs)
 
     return msg