`SafeChildWatcher` is deprecated in Python 3.14

[rmartin16]

Describe the bug

It appears the whole concept of "child watchers" was removed from Python.

    from rubicon.objc.eventloop import CocoaLifecycle, EventLoopPolicy
  File "/Users/user/.pyenv/versions/briefcase-3.14-dev/lib/python3.14/site-packages/rubicon/objc/eventloop.py", line 5, in <module>
    from asyncio import (
    ...<6 lines>...
    )
ImportError: cannot import name 'SafeChildWatcher' from 'asyncio' (/Users/user/.pyenv/versions/3.14-dev/lib/python3.14/asyncio/__init__.py)

Steps to reproduce

Attempt to start the event loop using Python 3.14.

Expected behavior

Event loop can start on Python 3.14.

Screenshots

No response

Environment

• Operating System: Sonoma
• Python version: 3.14.0a0
• Software versions:
  • Rubicon-ObjC: 0.4.9

Logs

No response

Additional context

This issue may more broadly capture the need to support Python 3.14 but I am not sure what else would be needed beyond resolving this child watcher issue.

RE: https://github.com/beeware/gbulb/issues/171

[freakboy3742]

I'm surprised this hasn't been surfaced as a deprecation warning in 3.12 and 3.13 testing... but it's definitely something that needs to be addressed.

[rmartin16]

It may just be that nothing is exercising this code in our use-cases.

EventLoopPolicy is implementing asyncio.events.AbstractEventLoopPolicy which used to require get_child_watcher() and is the only caller (AFAICT) of _init_watcher()...but neither the testing for Rubicon-ObjC nor Toga actually uses this.

At this point, AbstractEventLoopPolicy only requires get_event_loop(), set_event_loop(), and new_event_loop(); so, maybe it's reasonable to just only define methods for get_child_watcher() and set_child_watcher() on versions before 3.14.