gh-88965: typing: fix type substitution of a list of types after initial `ParamSpec` substitution (#102808)

Previously, this used to fail:

```py
from typing import *
T = TypeVar("T")
P = ParamSpec("P")

class X(Generic[P]):
    f: Callable[P, int]

Y = X[[int, T]]
Z = Y[str]
```

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>