Revert "python:3.6-windowservercore" update

https://github.com/docker-library/official-images/pull/2151#issuecomment-247659104

library/python

@@ -106,7 +106,7 @@ Tags: 3.6.0b1-onbuild, 3.6-onbuild
 GitCommit: 635ea5d58b53d165f7bedae90f8933c720a58150
 Directory: 3.6/onbuild
 
-Tags: 3.6.0b1-windowsservercore, 3.6-windowsservercore
-GitCommit: a70c6819aa38d938d56e96408aa6e7c3a9162c64
+Tags: 3.6.0a4-windowsservercore, 3.6-windowsservercore
+GitCommit: 855b85c8309e925814dfa97d61310080dcd08db6
 Directory: 3.6/windows/windowsservercore
 Constraints: windowsservercore