Fix synced itertools

[p-sawicki]

Fixes #20697

Changed the base type of itertools.batched from Iterator[tuple[_T_co, ...]] to Iterator[_T_co] in the stub and patch files so that mypy no longer reports an error due to a mismatched return type of __next__.

[github-actions]

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

openlibrary (https://github.com/internetarchive/openlibrary)
- openlibrary/solr/data_provider.py: note: In member "preload_metadata" of class "DataProvider":
- openlibrary/solr/data_provider.py:214: error: Argument 1 to "_get_lite_metadata" of "DataProvider" has incompatible type "tuple[tuple[str, ...], ...]"; expected "Sequence[str]"  [arg-type]
- openlibrary/solr/data_provider.py:224: error: Argument 1 to "_get_lite_metadata_direct" of "DataProvider" has incompatible type "tuple[str, ...]"; expected "str"  [arg-type]

[JelleZijlstra]

Why does this need to be fixed here and not in typeshed?

[ilevkivskyi]

@JelleZijlstra IIRC typeshed doesn't want to have explicit protocol bases. This is why we used to have the original patch in the first place. TBH I don't remember why we can't fix this in mypy (but at least this will add some perf penalty, since protocols are a bit slower than nominal subclasses).

[ilevkivskyi]

Oh, wait, now there is an actual inconsistency in the type of __next__(), yeah, this looks fishy and should be reported to typeshed.

[ilevkivskyi]

Oh, sorry, but the inconsistency is caused by our patch, all looks good then, ignore my previous comment.

[ilevkivskyi]

Yeah, I checked discussion on typeshed, they are aware this will be a breaking change, but the decision was that this is fine, since this is a relatively new feature (Python 3.12), and this will allow more precise item types.