tkinter crashes on macOS if imported before double-forking

[geofft]

On macOS, the following program crashes with our builds, but works as expected with /usr/bin/python3:

import os
import tkinter
if os.fork() == 0:
    os.fork()
    print("hi")

The child dies with this backtrace:

Thread 0 Crashed::  Dispatch queue: com.apple.main-thread
0   libsystem_platform.dylib      	       0x192243afc _os_unfair_lock_corruption_abort + 88
1   libsystem_platform.dylib      	       0x19223e3c0 _os_unfair_lock_lock_slow + 332
2   libtcl8.6.dylib               	       0x10326c7f8 AtForkPrepare + 44
3   libsystem_pthread.dylib       	       0x19220630c _pthread_atfork_prepare_handlers + 92
4   libSystem.B.dylib             	       0x1a044a178 libSystem_atfork_prepare + 32
5   libsystem_c.dylib             	       0x1920ba3a4 fork + 36
6   libpython3.13.dylib           	       0x104773e1c os_fork_impl + 120
7   libpython3.13.dylib           	       0x1041d91d8 cfunction_vectorcall_NOARGS.llvm.16321590194594904093 + 100
8   libpython3.13.dylib           	       0x103f47b40 _PyEval_EvalFrameDefault + 40532

and this app-specific information in the crash log:

{
  "CoreFoundation":["*** single-threaded process forked ***"],
  "libsystem_platform.dylib":["BUG IN CLIENT OF LIBPLATFOR
M: os_unfair_lock is corrupt","Abort Cause 258"],
  "libsystem_c.dylib":["crashed on child side of fork pre-exec"]
}

Deferring the tkinter import until after forking fixes it.

I feel like I saw this in one of the other threads I was staring at, probably in another project? intentionally avoiding importing tkinter as far as possible? And see also python/cpython#77292. I'd be tempted to say this isn't our fault except clearly it works fine on system Python.

Originally posted by @reteps in #146