Fix _ParentType annotation

Fix definition of `_ParentType` so that runtime type checkers work with nested Flax modules.

flax/linen/module.py

@@ -2901,7 +2901,7 @@ def module_paths(
     return {'/'.join(row.path): row.module_copy for row in table}
 
 
-_ParentType = Union[type[Module], Scope, type[_Sentinel], None]
+_ParentType = Union[Module, Scope, _Sentinel, None]
 
 
 def merge_param(name: str, a: T | None, b: T | None) -> T: