Update changelog.txt for release v3.8.0-slp

(cherry picked from commit abcfe37)

Author: Anselm Kruis

Stackless/changelog.txt

@@ -4,10 +4,10 @@ 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?
+What's New in Stackless 3.8.0?
 ==============================
 
-*Release date: 20XX-XX-XX*
+*Release date: 2021-08-11*
 
 - https://github.com/stackless-dev/stackless/issues/300
   Assignments to 'tasklet.trace_function', 'tasklet.profile_function' now raise