Update Lib free-threading core API tests to Python 3.15 - #9428

Summary

• Use scripts/update_lib migrate to add the canonical Python 3.15 free-threading package initializer and eighteen reviewed API test modules: bisect, exceptions, functions, functools, threading, types, string template races, methodcaller, enumerate, reversed, zip, itertools combinations/product, context, function annotations, list mutations, suggestions, system settings, and regular expressions.
• All nineteen files (991 lines) are byte-identical to CPython 3.15.0 d220409a13b77de7a6c30abe16ea90db01528edc. No test edits, RustPython skips, native changes, or dependency changes.
• This is a reviewed subset, not all free-threading tests. test_collections and test_csv remain deferred: deque mutation errors and CSV reader generation errors differ from canonical concurrency expectations. Their CPython critical-section semantics need separate native work; simply locking across arbitrary user callbacks risks reentrant deadlocks.

Runtime scope and limitations

RustPython advertises Py_GIL_DISABLED=1, so the canonical package gate does not skip these tests. Existing APIs and native locking paths were source-reviewed, but concurrency behavior has not been executed or proven compatible locally. There are 44 class test methods (approximately 46 cases by static inheritance, not runtime discovery; the combinatoric module's top-level test_concurrent_iteration function is only a helper).

Most workers have fixed loops or drain finite immutable inputs. List append/extend readers instead depend on successful completion of their 5,000-item writer; failure of that writer may strand readers. The list group has up to 11 simultaneous workers, and its reader iteration count is scheduling-dependent. The context writer is event-terminated: readers set the event in finally, and start_threads also calls its unlock callback to set it. Barriers and several direct joins/executor waits have no individual timeout. The existing native CI regrtest --timeout 600 provides outer subprocess containment, not a guarantee of termination, deadlock freedom, or runtime success.

Coverage is deliberately described narrowly: zip primarily checks concurrent exhaustion, not atomic tuple pairing or ordering; enumerate does not assert item/index pairing; combinations tests r=2, not the separate r=0 state path. Passing these tests would not prove general concurrent iterator atomicity. The regex scanner regression permits executions without a concurrent-entry error and checks neither ordering nor match counts: RustPython still lacks CPython's scanner execution guard, so this does not establish scanner concurrency equivalence. The canonical integer-string-limit test captures the final loop variable in its lambdas, so it exercises simultaneous setters rather than mixed getters/setters; the dlopenflags test retains its canonical capability skip. Those broader native semantics remain separate compatibility work.

Validation

• Canonical Git-blob equality for all nineteen paths and static AST/native-source review, including independent review.
• PYTHONPATH=scripts <CPython 3.15 host> -m unittest discover -s scripts/update_lib/tests: 228 passed after the four-file extension.
• Applicable repository hooks passed on all nineteen files (merge-conflict check, ruff format/check, redundant test patch check); git diff --check passed.
• No local concurrency test execution, native build, or full interpreter test run. Hosted CI is pending.