py3: call SMTP.docmd() with an str

Otherwise, this always returns (502, '5.5.2 Error: command not
recognized').

mercurial/mail.py

@@ -52,7 +52,7 b' class STARTTLS(smtplib.SMTP):'
         if not self.has_extn("starttls"):
             msg = b"STARTTLS extension not supported by server"
             raise smtplib.SMTPException(msg)
-        (resp, reply) = self.docmd(b"STARTTLS")
+        (resp, reply) = self.docmd("STARTTLS")
         if resp == 220:
             self.sock = sslutil.wrapsocket(
                 self.sock,