Django2 bytestring conversion: Lynx html_to_text output (#23558)

Fixes test of email body for Django 2.0.

openedx/core/lib/html_to_text.py

@@ -25,4 +25,4 @@ def html_to_text(html_message):
     if err_from_stderr:
         log.info(err_from_stderr)
 
-    return plaintext
+    return plaintext.decode()