[3.15] gh-151640: Track sharing of BytesIO buffer in free-threaded builds (GH-151651)

[ZeroIntensity]

(cherry picked from commit 881a13a)

• Issue: Data race between resize_buffer_lock_held and _Py_DecRefShared on a shared BytesIO buffer #151640

[python-cla-bot]

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[ZeroIntensity]

@pedramkarimii Could you resign the CLA using your GitHub account email?

[pedramkarimii]

@ZeroIntensity Done, thank you. I re-signed the CLA using my GitHub account email.

[pedramkarimii]

I investigated the failing free-threaded checks. In the 3.15 backport, the new regression test calls BytesIO.peek(), which is not available on that branch. This raises AttributeError in the reader threads and causes regrtest to report ENV CHANGED.

The observed failures are caused by the test incompatibility rather than the COW change itself. The backport test needs the peek() call removed. I can prepare a small follow-up patch for that adjustment if that would be helpful.