[3.12] gh-105430: Remove typing._Immutable unused internal helper (GH-105434) (#105451)

gh-105430: Remove `typing._Immutable` unused internal helper (GH-105434)
(cherry picked from commit 18309ad)

Co-authored-by: Nikita Sobolev <[email protected]>

Author: miss-islington

Lib/typing.py

@@ -425,17 +425,6 @@ def __init_subclass__(cls, /, *args, **kwds):
         if '_root' not in kwds:
             raise TypeError("Cannot subclass special typing classes")
 
-class _Immutable:
-    """Mixin to indicate that object should not be copied."""
-
-    __slots__ = ()
-
-    def __copy__(self):
-        return self
-
-    def __deepcopy__(self, memo):
-        return self
-
 
 class _NotIterable:
     """Mixin to prevent iteration, without being compatible with Iterable.