Replace accepts expression as string (#671)

Fixes #503

django-stubs/db/models/functions/text.pyi

@@ -35,7 +35,9 @@ class Repeat(BytesToCharFieldConversionMixin, Func):
     def __init__(self, expression: Union[Value, str], number: Optional[Union[Length, int]], **extra: Any) -> None: ...
 
 class Replace(Func):
-    def __init__(self, expression: Combinable, text: Value, replacement: Value = ..., **extra: Any) -> None: ...
+    def __init__(
+        self, expression: Union[Combinable, str], text: Value, replacement: Value = ..., **extra: Any
+    ) -> None: ...
 
 class Right(Left): ...
 class RPad(LPad): ...