PEP 695: __type_params__ attribute is writable

Fixes python/cpython#104600.

We realized that the attribute should be writable on functions and
classes for the benefit of wrappers such as @functools.wraps.

peps/pep-0695.rst

@@ -602,7 +602,7 @@ is a generator or coroutine.
 Accessing Type Parameters at Runtime
 ------------------------------------
 
-A new read-only attribute called ``__type_params__`` is available on generic classes,
+A new attribute called ``__type_params__`` is available on generic classes,
 functions, and type aliases. This attribute is a tuple of the
 type parameters that parameterize the class, function, or alias.
 The tuple contains ``TypeVar``, ``ParamSpec``, and ``TypeVarTuple`` instances.