`IntFloatTuple` from PEP 646 doesn't work as expected on Python<3.11

[Fatal1ty]

I was working on serialization / deserialization logic for the new PEP 646 in mashumaro, but unfortunately the following example from https://peps.python.org/pep-0646/#type-arguments-can-be-variadic doesn't work as expected on older python versions with typing-extensions 4.4.0:

from typing import Tuple, TypeVar
from typing_extensions import TypeVarTuple, Unpack, get_args


K = TypeVar("K")
T = TypeVar("T")
Ts = TypeVarTuple("Ts")

Ts1 = TypeVarTuple("Ts1")
Ts2 = TypeVarTuple("Ts2")
IntTuple = Tuple[int, Unpack[Ts1]]
IntFloatTuple = IntTuple[float, Unpack[Ts2]]

print(IntFloatTuple.__args__)
print(get_args(IntFloatTuple))

The PEP said:

Here, *Ts1 in the IntTuple alias is bound to Tuple[float, *Ts2], resulting in an alias IntFloatTuple equivalent to Tuple[int, float, *Ts2].

On python 3.11.0 it works seamlessly as expected:

print(IntFloatTuple.__args__)
# (<class 'int'>, <class 'float'>, *Ts2)

On python 3.10.9 it produces error on IntFloatTuple = IntTuple[float, Unpack[Ts2]]:

Traceback (most recent call last):
  File "/Users/alexander.tikhonov/projects/mashumaro/pep_646.py", line 12, in <module>
    IntFloatTuple = IntTuple[float, Unpack[Ts2]]
  File "/Users/alexander.tikhonov/.pyenv/versions/3.10.9/lib/python3.10/typing.py", line 309, in inner
    return cached(*args, **kwds)
  File "/Users/alexander.tikhonov/.pyenv/versions/3.10.9/lib/python3.10/typing.py", line 1070, in __getitem__
    arg = subst[arg]
KeyError: typing_extensions.Unpack[Ts1]

On python 3.9.16 it produces the same error on IntFloatTuple = IntTuple[float, Unpack[Ts2]]:

Traceback (most recent call last):
  File "/Users/alexander.tikhonov/projects/mashumaro/pep_646.py", line 12, in <module>
    IntFloatTuple = IntTuple[float, Unpack[Ts2]]
  File "/Users/alexander.tikhonov/.pyenv/versions/3.9.16/lib/python3.9/typing.py", line 274, in inner
    return cached(*args, **kwds)
  File "/Users/alexander.tikhonov/.pyenv/versions/3.9.16/lib/python3.9/typing.py", line 774, in __getitem__
    arg = subst[arg]
KeyError: typing_extensions.Unpack[Ts1]

On python 3.8.16 it produces wrong result:

print(IntFloatTuple.__args__)
(<class 'int'>, typing_extensions.Unpack[Ts1])
print(get_args(IntFloatTuple))
# (<class 'int'>, typing_extensions.Unpack[Ts1])

On python 3.7.16 it produces the same wrong result:

print(IntFloatTuple.__args__)
# (<class 'int'>, typing_extensions.Unpack[Ts1])
print(get_args(IntFloatTuple))
# (<class 'int'>, typing_extensions.Unpack[Ts1])

I couldn't figure out how to get type arguments the same on all python versions, and so it stops me from full support for variadic generics. At the moment I have to skip some tests like this one on python<3.11.

P.S. This problem also affects IntFloatsTuple = IntTuple[Unpack[Tuple[float, ...]]].

[JelleZijlstra]

I added a reference to this limitation to the docs in #171. I'd still accept a PR fixing it.