Issue #15239: Make mkstringprep.py work again on Python 3.

diff --cc Misc/NEWS
index b6cf7c9e9ff5522ce75760e2b71c04ba3efbb6b0,3ad038f5de4b6e90aa7d22737a06079627f96502..cfddde9d604d04f8fb5f554232c7c10f08fcbd7a
--- 1/Misc/NEWS
--- 2/Misc/NEWS
+++ b/Misc/NEWS
@@@ -1787,9 -1325,6 +1787,11 @@@ Documentatio
  Tools/Demos
  -----------
  
++- Issue #15239: Make mkstringprep.py work again on Python 3.
++
 +- Issue #17028: Allowed Python arguments to be supplied to the Windows
 +  launcher.
 +
  - Issue #17156: pygettext.py now uses an encoding of source file and correctly
    writes and escapes non-ascii characters.