[ybai08]

Fixes #749.

uvloop called PyOS_BeforeFork() / PyOS_AfterFork_Parent() for every spawned process, and PyOS_AfterFork_Child() in the child whenever libuv forked. As a result, callbacks registered with os.register_at_fork() ran on every create_subprocess_exec() / subprocess_exec(), even though the child never re-enters the interpreter before exec(). CPython's _posixsubprocess (and so the default asyncio loop) only does this when a preexec_fn is passed, which is also what the os.register_at_fork() docs describe.

This change does the same:

• the child fork handler is installed, and PyOS_BeforeFork() / PyOS_AfterFork_Parent() are called, only when preexec_fn is set;
• the error pipe's write end is made inheritable only in that case, since the child handler is what closes it (otherwise it stays close-on-exec).

Without a preexec_fn the child no longer runs any Python code between fork() and exec(). The steps the child handler did in that case were already covered by libuv: it resets all signal dispositions to SIG_DFL in the child (uv__process_child_init, or posix_spawnattr_setsigdefault on macOS), and uv_loop_fork() is not needed before exec(). This is also what already happens on macOS when libuv uses posix_spawn(), since pthread_atfork handlers don't run there.

On macOS the before / after_in_parent hooks fired for every spawn; with pass_fds (which forces a real fork) after_in_child fired too.

test_process_at_fork_hooks runs a small script in a fresh interpreter (the hooks can't be unregistered). The script registers all three hooks and spawns a child with no extra arguments, with pass_fds, and with preexec_fn. The test checks that the hooks run only for preexec_fn, under both the uvloop and the asyncio test cases.

Testing (macOS arm64, CPython 3.12, built with --use-system-libuv against Homebrew libuv 1.53.0):

• With uvloop/handles/process.pyx reverted to master, Test_UV_Process.test_process_at_fork_hooks fails: AssertionError: Lists differ: ['before', 'after_in_parent'] != []. Test_AIO_Process passes.
• With the change, both pass.
• The suite was run with python -m unittest discover tests: 529 tests OK (40 skipped). With PYTHONASYNCIODEBUG=1, the only failure is test_fs_event_rename, which fails the same way on master.
• flake8 and mypy (tests/test_sourcecode.py) pass.

Not run: Linux, the free-threaded build, and the bundled-libuv build.