deprecate the asyncio child watchers system

[graingert]

Deprecate the child watchers system and the policy system in favour of
asyncio.Runner(loop_factory=asyncio.ProactorEventLoop/asyncio.SelectorEventLoop/uvloop.new_event_loop)
that would be deprecating:

asyncio.get_event_loop() # already deprecated unless the loop is running 
asyncio.set_event_loop()  # asyncio.set_event_loop(None) should probably be exempt
asyncio.get_event_loop_policy()
asyncio.set_event_loop_policy()  # asyncio.set_event_loop_policy(None) should probably be exempt
asyncio.set_child_watcher()
asyncio.get_child_watcher()  # `_make_subprocess_transport` will instead attempt to use os.open_pidfd and fallback to starting a thread

I'd also like to introduce a new API: asyncio.EventLoop implemented as:

if sys.platform == "win32":
    EventLoop = ProactorEventLoop
else:
    EventLoop = SelectorEventLoop

asyncio.new_event_loop() will issue a DeprecationWarning if the current policy is not the default policy, and then in 3 releases become an alias of asyncio.EventLoop

Originally posted by @graingert in #93896 (comment)

• PR: GH-94597: add deprecation warnings for subclassing AbstractChildWatcher #99386

• PR: gh-94597: add asyncio.EventLoop #110723

• PR: gh-94597: deprecate asyncio.set_event_loop_policy #110728

[graingert]

Some background to this proposed deprecation:

This is a more comprehensive version of #82772

@serhiy-storchaka proposed deprecating set_event_loop() in #93453 (comment)

But maybe we should first deprecate set_event_loop()? It will be a no-op now.

@asvetlov noted that
get_event_loop_policy().get_event_loop() was not deprecated by oversight in #83710 (comment)

IMHO, asyncio.set_event_loop() and policy.get_event_loop()/policy.set_event_loop() are not deprecated by oversight.

[gvanrossum]

We are in dire need of more asyncio experts. @1st1 this isn't urgent but would be nice to have your perspective in time to do this in 3.12.

[kumaraditya303]

For 3.12 IMO we should deprecate MultiLoopWatcher #82504 and others which have race condition and other issue. Once that's done we may deprecate the entire child watcher system but just removing MultiLoopWatcher would be a good start.

[graingert]

@kumaraditya303 #94648

[gvanrossum]

We discussed this at the sprint and we agree that there are many things wrong with the child watchers and the policy system.

Deprecating child watchers: @1st1 thinks these should be done per loop (like uvloop does), not globally by the policy. Much more discussion on this topic is already in #82772. Bottom line, we agree to deprecate it, details remain to be seen.

Deprecating policies: Yes please. The policies no longer serve a real purpose. Loops are always per thread, there is no need to have a "current loop" when no loop is currently running. The only thing we still need is a loop factory, so perhaps instead of an API for getting/setting a global "policy", we could have an API for getting/setting a global "loop factory".

I'm fine with the EventLoop alias (it ties up a loose end), but I recommend that the API for creating a new event loop (when not using runners) should be asyncio.new_event_loop(), not asyncio.EventLoop().

We should totally deprecate set_event_loop() (even with None argument). At that point we can make get_event_loop() an alias for get_running_loop() (or the other way around -- I prefer calling get_event_loop() :-).