Update Lib iterator and group concurrency tests to Python 3.15 - #9436

Summary

Sync the missing canonical Python 3.15 free-threading itertools and grp suites (11 test methods).

Both files are copied through scripts/update_lib migrate from CPython d220409a13b77de7a6c30abe16ea90db01528edc, byte-identical. No new markers, assertion changes, native changes, or duplicated package initializer.

Validation and scope

• Updater unit tests: 228 passed. Applicable configured hooks and diff checks passed.
• Selected native iterator state/reference lifetime and group-database APIs were independently source-reviewed. No local concurrency tests, NSS/provider operations, or native builds were run. Hosted CI must establish runtime results.
• Iterator tests mostly check exhaustion and unexpected errors, not ordered/exact results or general iterator atomicity. The tee tests permit RuntimeError as upstream does.
• The iterator suite starts 2,068 threads over its rounds, at most 10 simultaneously, and can consume about 3.4 million tee items. Existing CI's 600-second timeout is outer containment, not a fixed completion-time or deadlock guarantee.
• Source review found a possible initial-cache publication race in cycle; the same fetch-then-cache structure exists in canonical CPython. This is not established as a RustPython-specific difference, and the unchanged canonical test retains the ability to expose it.
• Group tests perform 20 full enumerations. Extended lookups are capped by an existing post-enumeration 1,000-entry skip, but enumeration itself has no fixed entry bound. NSS providers can block or perform network reads, and concurrent database changes can affect round-trip assertions. No provider settings are changed.
• RustPython passes the package GIL-disabled gate; ordinary threading and module/platform gates remain unchanged.