Python 3.15rc2 on macos x86_64 creates additional thread at startup

Bug report

Bug description:

Python 3.15rc2 (from the official installer on macos x86_64 (rosetta emulation, not confirmed for native x86 hardware) creates a second thread at startup, causing any attempt to use os.fork with deprecation warnings enabled to emit a warning. This does not occur on arm64 builds or on non-macos platforms (tested linux and windows, x86 and arm).

Reproduction steps

1. Install python 3.15rc2 from https://www.python.org/downloads/release/python-3150rc2. Tested environment: M1 Macbook Pro running macos 26.6.2. Also observed on whatever github's macos-15 runner gives you.
2. Run arch -arm64 python3.15 -Werror -c 'import os; os.fork()'. See that no output is produced.
3. Run arch -x86_64 python3.15 -Werror -c 'import os; os.fork()'. It prints

Traceback (most recent call last):
  File "<string>", line 1, in <module>
    import os; os.fork()
               ~~~~~~~^^
DeprecationWarning: This process (pid=98584) is multi-threaded, use of fork() may lead to deadlocks in the child.

Expected results

A freshly started interpreter should not be running more than one thread and should be able to fork without warning.

CPython versions tested on:

3.15

Operating systems tested on:

macOS

Run the two reported arch commands with Python 3.15rc2 and compare the macOS arm64 and x86_64 behavior around os.fork(). Trace interpreter startup and thread creation on the x86_64 build; done when a fresh interpreter has no extra thread and the reported fork deprecation warning is absent.