set release version and date

yet another beta - still too many changes and unmerged
low-level changes, so we cannot go to RCs yet.

docs/changes.rst

@@ -217,8 +217,8 @@ The best check that everything is ok is to run a dry-run extraction::
 Change Log
 ==========
 
-Version 1.2.0xx (not released yet)
-----------------------------------
+Version 1.2.0b3 (2ß21-05-12)
+----------------------------
 
 Please note: