py3: use socket.makefile() instead of dropped smtplib.SSLFakeFile

The latter (undocumented internal) class got removed in Python 3.3. Use
socket.makefile() as suggested:

  https://docs.python.org/3/whatsnew/3.3.html#porting-python-code

mercurial/mail.py

@@ -61,7 +61,7 b' class STARTTLS(smtplib.SMTP):'
                 ui=self._ui,
                 serverhostname=self._host,
             )
-            self.file = smtplib.SSLFakeFile(self.sock)
+            self.file = self.sock.makefile("rb")
             self.helo_resp = None
             self.ehlo_resp = None
             self.esmtp_features = {}