Update changelog.txt for release v3.6.10-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 3.6.9?
-==============================
+What's New in Stackless 3.6.9 and 3.6.10?
+=========================================
 
-*Release date: 2019-08-03*
+*Release date: 2021-04-18*  (3.6.10)
+*Release date: 2019-08-03*  (3.6.9)
 
 - https://github.com/stackless-dev/stackless/issues/220
   Improve the error handling in case of failed stack transfers / hard tasklet