closes bpo-34525: Fix smtplib's authobject() documentation (GH-8965)

(cherry picked from commit 78deb7f33227972987722bc3fed5bcb45fae869e)

Co-authored-by: Sebastian Rittau <srittau@rittau.org>
diff --git a/Lib/smtplib.py b/Lib/smtplib.py
index 048c6bf..5e1bc0b 100755
--- a/Lib/smtplib.py
+++ b/Lib/smtplib.py
@@ -615,7 +615,7 @@
 
         It will be called to process the server's challenge response; the
         challenge argument it is passed will be a bytes.  It should return
-        bytes data that will be base64 encoded and sent to the server.
+        an ASCII string that will be base64 encoded and sent to the server.
 
         Keyword arguments:
             - initial_response_ok: Allow sending the RFC 4954 initial-response