bpo-46943: fix[imaplib]: call Exception with string instance

Adjust the behavior similar to `authenticate()` where self.error is
called with a str() instance.

Especially for Python3 with strict bytes mode (-bb) this is helpful and
prevents:

Traceback (most recent call last):
  in "<stdin>"
    self.login(email, password)
  File "/usr/lib/python3.7/imaplib.py", line 598, in login
    raise self.error(dat[-1])
imaplib.error: <exception str() failed>

During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  in "<stdin>"
    str(exc)
BytesWarning: str() on a bytes instance

Author: spaceone

Lib/imaplib.py

@@ -708,7 +708,7 @@ def login(self, user, password):
         """
         typ, dat = self._simple_command('LOGIN', user, self._quote(password))
         if typ != 'OK':
-            raise self.error(dat[-1])
+            raise self.error(dat[-1].decode('UTF-8', 'replace'))
         self.state = 'AUTH'
         return typ, dat
 

Lib/test/test_imaplib.py

@@ -415,6 +415,16 @@ def cmd_AUTHENTICATE(self, tag, args):
                 r'\[AUTHENTICATIONFAILED\] invalid'):
             client.authenticate('MYAUTH', lambda x: b'fake')
 
+    def test_invalid_login(self):
+        class MyServer(SimpleIMAPHandler):
+            def cmd_LOGIN(self, tag, args):
+                self.server.logged = args[0]
+                self._send_tagged(tag, 'NO', '[LOGIN] failed')
+        client, _ = self._setup(MyServer)
+        with self.assertRaisesRegex(imaplib.IMAP4.error,
+                r'\[LOGIN\] failed'):
+            client.login('user', 'wrongpass')
+
     def test_valid_authentication_bytes(self):
         class MyServer(SimpleIMAPHandler):
             def cmd_AUTHENTICATE(self, tag, args):