Allow objects matching SupportsKeysAndGetItem to be unpacked

[bryanforbes]

Fixes #14986

This PR allows any object matching _typeshed.SupportsKeysAndGetItem[str, Any] to be unpacked with **.

[bryanforbes]

@hauntsaninja I've got all but one unit test passing and the problem isn't obvious to me:

https://github.com/bryanforbes/mypy/blob/197916b1d357007c03a95c51df64feb582f5526f/test-data/unit/fine-grained-modules.test#L1280-L1309

The test reports that b, c, d, e, f, g, and h are all stale, so I've tried changing line 1300 to [stale b, c, d, e, f, g, h], but the next time I run the test, it tells me that a, b, c, d, e, f, g, and h are now stale. If I add a to the [stale] list, it goes back to reporting b through h as stale. Any help would be appreciated.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

prefect (https://github.com/PrefectHQ/prefect)
- src/prefect/infrastructure/process.py:235: error: List item 0 has incompatible type "object"; expected "Mapping[<nothing>, <nothing>]"  [list-item]
+ src/prefect/infrastructure/process.py:235: error: List item 0 has incompatible type "object"; expected "SupportsKeysAndGetItem[<nothing>, <nothing>]"  [list-item]

hydra-zen (https://github.com/mit-ll-responsible-ai/hydra-zen)
- src/hydra_zen/structured_configs/_make_custom_builds.py:299: error: List item 0 has incompatible type "Optional[DataclassOptions]"; expected "Mapping[Any, Any]"  [list-item]
+ src/hydra_zen/structured_configs/_make_custom_builds.py:299: error: List item 0 has incompatible type "Optional[DataclassOptions]"; expected "SupportsKeysAndGetItem[Any, Any]"  [list-item]

[hauntsaninja]

Thanks, this looks great!

I don't understand what's going on with testAddModuleAfterCache3 an don't understand why adding two files fixes. But the core logic here seems good and unit tests should fail if typeshed ever makes enough changes that e.g. the protocol no longer exists or _typeshed is not in the builtins scc or whatever