macOS: every mutant reports segfault when tests touch urllib.request.getproxies() (SystemConfiguration is not fork-safe) #618

Environment: mutmut 3.8.1 (also current main), Python 3.12 and 3.14 (Homebrew), macOS 27 on Apple Silicon.

What happens: if the code under test or the tests call urllib.request.getproxies() (which requests, httpx and aiohttp do when they create a client), every mutant is reported as segfault. macOS shows a crash report for each child. This is not #446: use_setproctitle already defaults to off on macOS.

Minimal reproduction:

src/demo/__init__.py

def is_adult(age):
    return age >= 18

tests/test_demo.py

import urllib.request
from demo import is_adult

def test_is_adult():
    urllib.request.getproxies()   # what HTTP clients do on setup
    assert is_adult(18)
    assert not is_adult(17)

setup.cfg

[mutmut]
source_paths=src/demo
pythonpath=src

$ mutmut run && mutmut results --all true
    demo.x_is_adult__mutmut_1: segfault
    demo.x_is_adult__mutmut_2: segfault

Cause: with no *_proxy environment variable set, getproxies() on macOS falls back to _scproxy.get_proxies(), which calls SCDynamicStoreCopyProxiesWithOptions (SystemConfiguration / CoreFoundation). That code isn't safe in a forked child. The crash reports show two shapes, both ending in "crashed on child side of fork pre-exec":

• SIGABRT from performForkChildInitialize (objc +initialize after fork)
• SIGSEGV in os_log_type_enabled under CFPreferences, with "multi-threaded process forked"

Both stacks go through get_proxies → SCDynamicStoreCopyProxiesWithOptions. Background: CPython bpo-31818.

Workaround: no_proxy='*' mutmut run. That gives killed for both mutants.

Proposed fix (#619): query the system proxy settings once in the main process, before forking, and replace urllib's two _scproxy lookups with the cached answers. Forked children inherit them, so proxy and bypass results stay identical and the environment isn't changed.