gh-101277: Isolate itertools, add group and _grouper types to module state by erlend-aasland · Pull Request #101302 · python/cpython

This is part 1 of 4. I've split each PR so the diffs are ~200 lines of code, excluding clinic changes. This PR add helpers for fetching state, adds tests for type immutability, converts two of the types to heap types, and add them to the module state.

Follow-up PRs for the rest of the types are created as draft PRs; these will be rebased onto main after this PR has landed.

The last PR will purge the global-to-fix file, and add a NEWS entry.