Fix mypy error and add changelog entry

Use ClassVar for inheritable on both ClassDecoratorSetting and Union to
satisfy mypy. Add unreleased changelog entry for #80.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: NiklasRosenstein

.changelog/_unreleased.toml

@@ -0,0 +1,5 @@
+[[entries]]
+id = "f3a8c1d2-7e4b-4a9f-b6c5-2d1e8f9a0b3c"
+type = "fix"
+description = "Fix #80: Union setting inherited via MRO causing RecursionError during serialization/deserialization"
+author = "@NiklasRosenstein"

databind/src/databind/core/settings.py

@@ -160,7 +160,7 @@ def __init__(self) -> None:
 
 class ClassDecoratorSetting(Setting):
     bound_to: t.Optional[type] = None
-    inheritable: bool = True
+    inheritable: t.ClassVar[bool] = True
 
     def __init__(self) -> None:
         if type(self) is ClassDecoratorSetting: