dataclass.replace: allow transformed classes (#15915)

We [already
synthesize](#15843 (comment))
`__dataclass_fields__` for all classes including
`@dataclass_transform`'d ones, thus assume more than PEP-681 says. We
might as well assume `dataclasses.replace` applies on all same classes.
This way we risk false positive since it'll raise in runtime.

Fixes #15843.

mypy/plugins/dataclasses.py

@@ -375,9 +375,7 @@ def transform(self) -> bool:
             add_attribute_to_class(self._api, self._cls, "__match_args__", match_args_type)
 
         self._add_dataclass_fields_magic_attribute()
-
-        if self._spec is _TRANSFORM_SPEC_FOR_DATACLASSES:
-            self._add_internal_replace_method(attributes)
+        self._add_internal_replace_method(attributes)
         if "__post_init__" in info.names:
             self._add_internal_post_init_method(attributes)