mailman: cope with mails containing broken unicode

See https://pagure.io/fedora-infrastructure/issue/8824 for details. This
enables the hotfix on staging only to enable testing.

Signed-off-by: Nils Philippsen <nils@redhat.com>

files/hotfix/mailman3/mailman3-broken-unicode-emails.patch

@@ -0,0 +1,19 @@
+--- a/src/mailman/email/message.py
++++ b/src/mailman/email/message.py
+@@ -47,6 +47,16 @@ class Message(email.message.Message):
+     def __setstate__(self, values):
+         self.__dict__ = values
+
++    def as_string(self):
++        # Work around for https://bugs.python.org/issue27321 and
++        # https://bugs.python.org/issue32330.
++        try:
++            value = email.message.Message.as_string(self)
++        except (KeyError, UnicodeEncodeError):
++            value = email.message.Message.as_bytes(self).decode(
++                'ascii', 'replace')
++        return value
++
+     @property
+     def sender(self):
+         """The address considered to be the author of the email.