Make types.UnionType.__getitem__ parameter positional-only

[brianschubert]

New hit from running stubtest with python/mypy#19593, introduced in #14687

For reference:

>>> (int | list[T]).__getitem__(parameters=int)
Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    (int | list[T]).__getitem__(parameters=int)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^
TypeError: wrapper __getitem__() takes no keyword arguments

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉