py3: call SMTP.has_extn() with an str

Passing a bytes on Python 3 always returns False, thus starttls is not
properly detected.

mercurial/mail.py

@@ -49,7 +49,7 b' class STARTTLS(smtplib.SMTP):'
         self._host = host
 
     def starttls(self, keyfile=None, certfile=None):
-        if not self.has_extn(b"starttls"):
+        if not self.has_extn("starttls"):
             msg = b"STARTTLS extension not supported by server"
             raise smtplib.SMTPException(msg)
         (resp, reply) = self.docmd(b"STARTTLS")