Issue #23779: imaplib raises TypeError if authenticator tries to abort.

Patch from Craig Holmquist.
diff --git a/Lib/imaplib.py b/Lib/imaplib.py
index 4d9df55..eb05dcb 100644
--- a/Lib/imaplib.py
+++ b/Lib/imaplib.py
@@ -1306,7 +1306,7 @@
     def process(self, data):
         ret = self.mech(self.decode(data))
         if ret is None:
-            return '*'      # Abort conversation
+            return b'*'     # Abort conversation
         return self.encode(ret)
 
     def encode(self, inp):