Fix subclassing message for 3.10- and instance subclassing

Author: JanEricNitschke

src/typing_extensions.py

@@ -2634,13 +2634,19 @@ def _typevartuple_prepare_subst(alias, args):
                 )
 
             tvt.__typing_prepare_subst__ = _typevartuple_prepare_subst
+
+            def __mro_entries__(self, bases):
+                raise TypeError("Cannot subclass an instance of TypeVarTuple.")
+            tvt.__mro_entries__ =  __mro_entries__
+
             return tvt
 
         def __init_subclass__(self, *args, **kwds):
             raise TypeError(
                 f"type '{__name__}.TypeVarTuple' is not an acceptable base type"
             )
 
+
 else:  # <=3.10
     class TypeVarTuple(_DefaultMixin):
         """Type variable tuple.
@@ -2717,7 +2723,12 @@ def __reduce__(self):
 
         def __init_subclass__(self, *args, **kwds):
             if '_root' not in kwds:
-                raise TypeError("Cannot subclass special typing classes")
+                raise TypeError(
+                    f"type '{__name__}.TypeVarTuple' is not an acceptable base type"
+                )
+
+        def __mro_entries__(self, bases):
+            raise TypeError("Cannot subclass an instance of TypeVarTuple.")
 
 
 if hasattr(typing, "reveal_type"):  # 3.11+