py3: decode username and password before SMTP login

smtplib.SMTP.login() requires str on Python 3.

For 'password', we only need to decode when value comes from config as
getpass() returns the correct type already.

mercurial/mail.py

@@ -145,10 +145,14 b' def _smtp(ui):'
         sslutil.validatesocket(s.sock)
     username = ui.config(b'smtp', b'username')
     password = ui.config(b'smtp', b'password')
-    if username and not password:
+    if username:
+        if password:
+            password = encoding.strfromlocal(password)
+        else:
         password = ui.getpass()
     if username and password:
         ui.note(_(b'(authenticating to mail server as %s)\n') % username)
+        username = encoding.strfromlocal(username)
         try:
             s.login(username, password)
         except smtplib.SMTPException as inst: