rcmail: pass smtplib.SMTP.sendmail to_addrs as list

Passing it as a set worked ... but is apparently wrong. The documentation states
it has to be a "list of addresses".

Pytype warned:

File "kallithea/lib/rcmail/smtp_mailer.py", line 99, in send: Function SMTP.sendmail was called with the wrong arguments [wrong-arg-types]
         Expected: (self, from_addr, to_addrs: Union[Sequence[str], str], ...)
  Actually passed: (self, from_addr, to_addrs: set, ...)

kallithea/lib/rcmail/smtp_mailer.py

@@ -96,7 +96,7 b' class SmtpMailer(object):'
         if self.user and self.passwd:
             smtp_serv.login(self.user, self.passwd)
 
-        smtp_serv.sendmail(msg.sender, msg.send_to, raw_msg.as_string())
+        smtp_serv.sendmail(msg.sender, list(msg.send_to), raw_msg.as_string())
         logging.info('MAIL SENT TO: %s' % recipients)
 
         try:

kallithea/tests/other/test_mail.py

@@ -21,9 +21,8 b' class smtplib_mock(object):'
 
     def sendmail(self, sender, dest, msg):
         smtplib_mock.lastsender = sender
-        smtplib_mock.lastdest = dest
+        smtplib_mock.lastdest = set(dest)
         smtplib_mock.lastmsg = msg
-        pass
 
 
 @mock.patch('kallithea.lib.rcmail.smtp_mailer.smtplib', smtplib_mock)