Don't turn list defaults for ParamSpecs into tuples

[AlexWaygood]

PEP 696 specifies that default values for ParamSpecs must be lists, and CPython on 3.13 keeps them as a list for the __default__ attribute:

Python 3.14.0a0 (heads/cleanup-typing:2e98d5dbf9, May 11 2024, 12:20:48) [Clang 15.0.0 (clang-1500.3.9.4)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> def foo[**P = [str, int]](): pass
... 
>>> foo.__type_params__[0].__default__
[<class 'str'>, <class 'int'>]
>>> 

Currently in the typing_extensions version, however, we turn any list into a tuple, which seems wrong:

(main) % python                                                                                                          ~/dev/typing_extensions/src
Python 3.12.3 (main, Apr 30 2024, 10:12:02) [Clang 15.0.0 (clang-1500.3.9.4)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from typing_extensions import ParamSpec
>>> ParamSpec("P", default=[int, str]).__default__
(<class 'int'>, <class 'str'>)

This PR fixes that.

[AlexWaygood]

The _type_check() calls here feel "morally correct" in a sense -- they're consistent with what a lot of the rest of the typing.py internals do. But on the other hand, CPython doesn't actually do any check at all in its 3.13 implementation; it just sets the __default__ attribute to the value passed in, and moves on.

[JelleZijlstra]

So maybe we should just do that here too?

[AlexWaygood]

I wasn't sure whether the bug was that we were being too lenient or that CPython wasn't being restrictive enough. But having slept on it, I now agree that we should just accept any default value, and not do any type checking here. That's consistent with what CPython has always done for TypeVar bounds and TypeVar constraints, which should also conceptually be types:

% pyenv local 3.11                                                                                                % python
Python 3.11.8 (main, Feb 15 2024, 19:32:18) [Clang 15.0.0 (clang-1500.1.0.2.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from typing import TypeVar
>>> def not_a_type(): pass
... 
>>> T = TypeVar("T", bound=not_a_type)
>>> U = TypeVar("U", not_a_type, not_a_type)

[JelleZijlstra]

So maybe we should just do that here too?