Matching generic return type of `SupportsGetItem`

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# Describe the bug
`pyright` does not match the generic return type of `SupportsGetItem` properly.

# Code or Screenshots

```python
from __future__ import annotations

from typing import TYPE_CHECKING, Any, TypeVar

if TYPE_CHECKING:
from _typeshed import SupportsGetItem

class Series: ...

class DataFrame:
def __getitem__(self, key: object) -> Series:
return Series()

T = TypeVar("T")

def f(x: SupportsGetItem[Any, T]) -> T:
if isinstance(x, DataFrame):
return Series()
else:
return x[0]

```

# VS Code extension or command-line

```text
Type "Series" is not assignable to return type "T@f"
Type "Series" is not assignable to type "T@f" (reportReturnType)
```

- `mypy` works fine: https://mypy-play.net/?gist=3aaea716bd3429a90a96457884f46712
- `pyright` reports an error: https://pyright-play.net/?strict=true&code=GYJw9gtgBA%2BjwFcAuCQFM5QJYQA5hCSgEMA7UsJYpLMUgZwFgAoF0SKJAT1y1IHNseAkQAqATQAKAURgBhABLS5AaQCSAOQDiAGigBBUlz2ieaAGrEQLFlmBQJM%2BUtWatALhZRvUdtBjcuGj0ABZoACZC%2BIRQAMoIuNFI9FpoSGpIaBA2zADGADbE9PRxaCBYwe5QAHS1OQVFJQAi1MQAYiDEEGiezD5Q4Wj2cPxpWJkQcAAU9Gj5wHoA1mhcVWAARgBWaLlIAJRQALQAfKXllV79PugoIKRnFfRTezmiUAC8DmaWIFMARKI-i9WMxBvZgFMAB5VeKJEQpNIZLIAbUMxgcAF0DicHL1%2BnZsPQ%2BPQqKRcmgoXoWlQOl00Hs8VdvDdUPdYmVHs9Lt45rNGUyWXcoJDkQAGDEsIA

Start by running the provided Python example in strict mode and comparing pyright's diagnostic with the linked mypy result. Trace how generic return types are inferred for SupportsGetItem and the DataFrame branch. Done means pyright accepts the example without incorrectly reporting that Series is incompatible with T.

Written by the indexing model from the issue text.