ext/mutable/MutableBase: fix coerce type (#256)

* tests: add test for issue #255

* ext/mutable/MutableBase: fix coerce type
Fixes #255

sqlalchemy-stubs/ext/mutable.pyi

@@ -11,7 +11,7 @@ from ..util import memoized_property as memoized_property
 
 class MutableBase:
     @classmethod
-    def coerce(cls, key: Any, value: Any) -> None: ...
+    def coerce(cls, key: str, value: Any) -> Any: ...
 
 class Mutable(MutableBase):
     def changed(self) -> None: ...

test/files/issue_255.py

@@ -0,0 +1,14 @@
+from __future__ import annotations
+
+from typing import Any
+
+from sqlalchemy.ext.mutable import Mutable
+
+
+class Issue255(Mutable):
+
+    @classmethod
+    def coerce(cls, key: str, value: Any) -> str | None:
+        if value is None:
+            return None
+        return str(value)