Make NewType.__call__ params positional-only

[AlexWaygood]

This is really minor, but it means that the signature of typing_extensions.NewType.__call__ exactly matches that of typing.NewType.__call__ on all Python versions we support:

Python 3.8.16 (default, Mar  2 2023, 03:18:16) [MSC v.1916 64 bit (AMD64)] :: Anaconda, Inc. on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> from typing import NewType
>>> x = NewType("x", int)
>>> x(obj=42)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: new_type() got an unexpected keyword argument 'obj'

Python 3.10.8 | packaged by conda-forge | (main, Nov 24 2022, 14:07:00) [MSC v.1916 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> x = NewType("x", int)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'NewType' is not defined
>>> from typing import NewType
>>> x = NewType("x", int)
>>> x(obj=42)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: NewType.__call__() got an unexpected keyword argument 'obj'

[AlexWaygood]

(I don't think this really needs a changelog entry, but I'm happy to add one if folks disagree!)

[JelleZijlstra]

Thanks! We should document this in the changelog as it is a user-visible change, but no need to mention it in the docs.