Update changelog.txt for release v3.8.1-slp

Author: Anselm Kruis

Stackless/changelog.txt

@@ -4,16 +4,11 @@ Stackless-Python News
 This file documents user visible changes to the Stackless extension of C-Python.
 For other changes see Misc/NEWS.
 
-What's New in Stackless 3.X.X?
-==============================
-
-*Release date: 20XX-XX-XX*
-
-
-What's New in Stackless 3.8.0?
-==============================
+What's New in Stackless 3.8.0 and 3.8.1?
+========================================
 
-*Release date: 2021-08-11*
+*Release date: 2021-08-11*  (3.8.1)
+*Release date: 2021-08-11*  (3.8.0)
 
 - https://github.com/stackless-dev/stackless/issues/300
   Assignments to 'tasklet.trace_function', 'tasklet.profile_function' now raise