PEP 499: Fix 404 mail.python.org link (#4245)

Co-authored-by: Hugo van Kemenade <[email protected]>

Author: srinivasreddy

peps/pep-0499.rst

@@ -229,7 +229,7 @@ References
 
 .. _issue 19702: http://bugs.python.org/issue19702
 
-.. _I tripped over this issue: https://mail.python.org/pipermail/python-list/2015-August/694905.html
+.. _I tripped over this issue: https://mail.python.org/pipermail/python-list/2015-August/845302.html
 
 
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