prepare the release (#101)

Author: keewis

docs/whats-new.rst

@@ -1,8 +1,8 @@
 What's new
 ==========
 
-(*unreleased*)
---------------
+2023.04.0 (2022-04-12)
+----------------------
 - add official support for python 3.11 and `sphinx>=5.0` (:pull:`87`)
 - change the policy to only actively support the last minor release of older major
   versions of `sphinx`.  Currently supported versions are now: `3.5`, `4.5`, and all minor