Match epoll.modify keyword and eventmask conversion contracts - #9554

Summary

Accept both fd and eventmask as positional or keyword arguments to select.epoll.modify, matching CPython 3.15.0's (self, /, fd, eventmask) signature.

Bind raw arguments before calling user code. After argument-shape validation, retain the order: file-descriptor conversion, event-mask __index__ conversion, any range warning, closed-epoll check, existing host operation. The mask uses the low 32 bits; values outside [-2**31, 2**32 - 1] emit DeprecationWarning("integer value out of range") at stack level 1. A warning promoted to an exception returns before the closed check or syscall. No descriptor ownership changes are introduced.

This changes only modify; it is independent of #9553 (unregister) and does not change register. The small mask helper is local to select; no shared-converter refactor is included. Existing shared Fildes differences, including the missing RuntimeWarning for a direct bool descriptor, are not addressed.

Reference

CPython 3.15.0 select Clinic wrapper and PyLong_AsNativeBytes.

Validation

• Independent source review, normal commit hooks and git diff --check passed.
• Added a pure Rust metadata test for both required parameters, arity, keyword-call flag and generated signature. It has not yet been compiled or executed locally.
• Offline workspace/C-API tests, Clippy and test_epoll attempts were blocked before compilation by the unavailable parking_lot_core git dependency. No local compilation or runtime pass is claimed.
• Existing Lib/test/test_epoll.py covers positional modify and closed-epoll behavior. It does not establish new keyword handling, mask boundaries, warning-as-error behavior or callback ordering by execution; those remain runtime validation gaps.
• No unsafe descriptor probes were run. No Lib tests or workflows were changed. Generic binder diagnostic wording and all combinations of malformed-call error priorities are not claimed identical to CPython.

Draft pending exact-head CI and its actual --signature report.

AI assistance: OpenAI Codex performed source investigation, implementation, metadata-test drafting and review.