Python 3.13.0b1: test_typing_extensions: `AttributeError: attribute '__default__' of 'typing.TypeVar' objects is not writable` about typing_extensions

[AlexWaygood]

@AlexWaygood the changes you made didn't look related to the test failure @befeleme pointed out here. Do you know what's going on here?

The test failure @befeleme pointed out was more than a test failure: the test module was failing at import time. The failure was because the __default__ attribute on typing.TypeVar is read-only on py313+: in our implementation of PEP-696, we were creating instances of typing.TypeVar, then trying to monkeypatch __default__ attributes onto the TypeVar instances after creating them. But it's unnecessary to monkeypatch attributes onto instances of TypeVar on Python 3.13+, because the "real" TypeVar implementation has a default parameter for its constructor on Python 3.13+. So I fixed the error in #378 by simply not independently reimplementing TypeVar at all on Python 3.13+; instead, we now just from typing import TypeVar on Python 3.13+.

[AlexWaygood]

#378 fixed all but one test failure. The last remaining one is to do with AsyncContextManager having an optional second parameter on Python 3.13, but not in the backported version that typing_extensions provides.

[JelleZijlstra]

@AlexWaygood the changes you made didn't look related to the test failure @befeleme pointed out here. Do you know what's going on here?

Also, there's another failure incoming due to changes I made for beta 2:

% ~/py/cpython/python.exe test_typing_extensions.py 
Traceback (most recent call last):
  File "/Users/jelle/py/typing_extensions/src/test_typing_extensions.py", line 42, in <module>
    from _typed_dict_test_helper import Foo, FooGeneric, VeryAnnotated
  File "/Users/jelle/py/typing_extensions/src/_typed_dict_test_helper.py", line 17, in <module>
    class FooGeneric(TypedDict, Generic[T]):
                                ~~~~~~~^^^
  File "/Users/jelle/py/cpython/Lib/typing.py", line 431, in inner
    return func(*args, **kwds)
  File "/Users/jelle/py/cpython/Lib/typing.py", line 1243, in _generic_class_getitem
    return _GenericAlias(cls, args)
  File "/Users/jelle/py/cpython/Lib/typing.py", line 1420, in __init__
    self.__parameters__ = _collect_parameters(
                          ~~~~~~~~~~~~~~~~~~~^
        args,
        ^^^^^
        enforce_default_ordering=enforce_default_ordering,
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
TypeError: _collect_parameters() got an unexpected keyword argument 'enforce_default_ordering'

Will work on a fix for that one.