* Sync ten foundational Python libraries with CPython 3.15
Migrate _py_abc, stat, genericpath, ntpath, posixpath, functools, gettext, string, optparse, and mimetypes with related canonical tests using scripts/update_lib from CPython d220409a13b77de7a6c30abe16ea90db01528edc. Preserve the existing pure LRU hash-count expected failure.
Assisted-by: OpenAI Codex:model version unavailable
* Preserve positional-only annotation order for singledispatch
Emit positional-only annotations before regular positional annotations, matching declaration order and the canonical functools regression coverage.
Assisted-by: OpenAI Codex:model version unavailable
Initially this is only meant to be a small update for `test_io.py`.
But it turns out that `test.support` need a lot of cleanup especially
for `test.support.warnings_helper`.
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