[3.13] gh-87135: Hang non-main threads that attempt to acquire the GIL during finalization (GH-105805) (GH-137827)

* [3.13] gh-87135: Hang non-main threads that attempt to acquire the GIL during finalization (GH-105805)

Instead of surprise crashes and memory corruption, we now hang threads that attempt to re-enter the Python interpreter after Python runtime finalization has started. These are typically daemon threads (our long standing mis-feature) but could also be threads spawned by extension modules that then try to call into Python. This marks the `PyThread_exit_thread` public C API as deprecated as there is no plausible safe way to accomplish that on any supported platform in the face of things like C++ code with finalizers anywhere on a thread's stack. Doing this was the least bad option.

(cherry picked from commit 8cc5aa47ee464ddfd8da5461edecf4a5c72df2ff)

Co-authored-by: Jeremy Maitin-Shepard <jeremy@jeremyms.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>

* state "3.13.7 and earlier"
* backport: do not add the deprecated marker
* fix Py_IsFinalizing doc ref

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Co-authored-by: Jeremy Maitin-Shepard <jeremy@jeremyms.com>