Update changelog for release 2.12.0

CHANGES.md

@@ -1,4 +1,4 @@
-2.12.0 (xxxx-xx-xx)
+2.12.0 (2023-05-12)
 -------------------
 
 * atdgen: Annotate generated code with types to disambiguate OCaml
@@ -11,6 +11,9 @@
   from `"None"` which both translate to `None` in Python. (#332)
 * (BREAKING) atdgen: revert default encoding of int64 values as string (#330)
 * atdgen: Support `<json repr="string">` for `int` values (#330)
+* atdpy: Treat default field values as expressions to evaluate each time 
+  they're assigned to a field. This allows the use of mutable defaults such as
+  lists (#339)
 
 2.11.0 (2023-02-08)
 -------------------