Update changelog.txt for release v3.7.7-slp

Author: akruis

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 3.7.6?
-==============================
+What's New in Stackless 3.7.6 and 3.7.7?
+========================================
 
-*Release date: 2021-05-20*
+*Release date: 2021-05-21*  (3.7.7)
+*Release date: 2021-05-20*  (3.7.6)
 
 - https://github.com/stackless-dev/stackless/issues/231
   Update Stackless related bitbucket.org URLs to github.com