PEP 539: Fix Python-Version header (GH-651)

Follow-up to GH-648

The genpepindex script requires the headers to be in a particular order.
In pep-0539.txt the order was different, breaking the build for everyone.

pep-0539.txt

@@ -4,11 +4,11 @@ Version: $Revision$
 Last-Modified: $Date$
 Author: Erik M. Bray, Masayuki Yamamoto
 BDFL-Delegate: Nick Coghlan
-Python-Version: 3.7
 Status: Final
 Type: Standards Track
 Content-Type: text/x-rst
 Created: 20-Dec-2016
+Python-Version: 3.7
 Post-History: 16-Dec-2016, 31-Aug-2017, 08-Sep-2017
 Resolution: https://mail.python.org/pipermail/python-dev/2017-September/149358.html