py3: decode payload of notify email

This fixes one UnicodeEncodeError in test-notify.t:422 when testing the
notify hook with non-ascii content (there are more later). We only
decode on Python 3, since it's not safe for sure on Python 2.

hgext/notify.py

@@ -400,7 +400,7 b' class notifier(object):'
             # create fresh mime message from scratch
             # (multipart templates must take care of this themselves)
             headers = msg.items()
-            payload = msg.get_payload()
+            payload = msg.get_payload(decode=pycompat.ispy3)
             # for notification prefer readability over data precision
             msg = mail.mimeencode(self.ui, payload, self.charsets, self.test)
             # reinstate custom headers