Fix assigning expressions to Mapped attributes. (#69)

Author: MaicoTimmerman

sqlalchemy-stubs/orm/attributes.pyi

@@ -38,6 +38,7 @@ from .. import event as event
 from .. import inspection as inspection
 from .. import util as util
 from ..sql import base as sql_base
+from ..sql import ClauseElement
 from ..sql import roles as roles
 from ..sql import traversals as traversals
 from ..sql import visitors as visitors
@@ -91,7 +92,10 @@ class Mapped(QueryableAttribute, Generic[_T]):
     def __get__(self, instance: None, owner: Any) -> "Mapped"[_T]: ...
     @overload
     def __get__(self, instance: object, owner: Any) -> _T: ...
+    @overload
     def __set__(self, instance: Any, value: _T) -> None: ...
+    @overload
+    def __set__(self, instance: Any, value: ClauseElement) -> None: ...
     def __delete__(self, instance: Any) -> None: ...
     @classmethod
     def _empty_constructor(cls, arg1: Any) -> "Mapped"[_T]: ...