Support for Union[*TypeVarTuple]

(Forwarded from https://github.com/python/typing/issues/1523)
I'm trying to annotate a method that only accepts a union of specific types, with the types specified via variadic generic. Although mypy supports variadic unpacking like Tuple[*Ts], unpacking Union[*Ts] doesn't seem to work yet:

import typing

Ts = typing.TypeVarTuple("Ts")

class Accepts(typing.Generic[*Ts]):
    # error: Unpack is only valid in a variadic position  [valid-type]
    def foo(self, x: typing.Union[*Ts]): ...

class A: ...
class B: ...
class C: ...
class Test(Accepts[A, B]): ...

Test().foo(A())  # ok
Test().foo(B())  # ok
Test().foo(C())  # should raise [arg-type] error

Mypy 1.7.1, Python 3.11

Start by reproducing the example with mypy 1.7.1 on Python 3.11 and confirm the current diagnostic for Union[*Ts]. Trace the existing variadic unpacking support from the reported valid-type error. Done means Union[*Ts] is accepted and the example permits A and B while rejecting C with an arg-type error.