mail: force SMTPException to bytes before wrapping in error.Abort

Pytype wasn't complaining about this for some reason, but PyCharm was and we
already handle another instance of this exception the same way in this function.

Differential Revision: https://phab.mercurial-scm.org/D10473

mercurial/mail.py

@@ -164,7 +164,7 b' def _smtp(ui):'
             recipients = [r[1] for r in inst.recipients.values()]
             raise error.Abort(b'\n' + b'\n'.join(recipients))
         except smtplib.SMTPException as inst:
-            raise error.Abort(inst)
+            raise error.Abort(stringutil.forcebytestr(inst))
 
     return send