GH-120804: Remove get_child_watcher and set_child_watcher from asyncio

[kumaraditya303]

This PR removes get_child_watcher and set_child_watcher from asyncio. At some place currently we directly write to policy._watcher as there are no apis to do that for testing. This is temporary and will change in subsequent PRs when subprocesses will be rewritten to not use child watchers at all and then this "hack" will become unnecessary and be removed.

• Issue: Rewrite asyncio subprocesses without child watchers  #120804

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Ubuntu NoGIL Refleaks 3.x has failed when building commit 96ead91.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/all/#builders/1226/builds/2089) and take a look at the build logs.
4. Check if the failure is related to this commit (96ead91) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/all/#builders/1226/builds/2089

Failed tests:

• test_free_threading

Summary of the results of the build (if available):

==

Click to see traceback logs

remote: Enumerating objects: 12, done.        
remote: Counting objects:   8% (1/12)        
remote: Counting objects:  16% (2/12)        
remote: Counting objects:  25% (3/12)        
remote: Counting objects:  33% (4/12)        
remote: Counting objects:  41% (5/12)        
remote: Counting objects:  50% (6/12)        
remote: Counting objects:  58% (7/12)        
remote: Counting objects:  66% (8/12)        
remote: Counting objects:  75% (9/12)        
remote: Counting objects:  83% (10/12)        
remote: Counting objects:  91% (11/12)        
remote: Counting objects: 100% (12/12)        
remote: Counting objects: 100% (12/12), done.        
remote: Compressing objects:  11% (1/9)        
remote: Compressing objects:  22% (2/9)        
remote: Compressing objects:  33% (3/9)        
remote: Compressing objects:  44% (4/9)        
remote: Compressing objects:  55% (5/9)        
remote: Compressing objects:  66% (6/9)        
remote: Compressing objects:  77% (7/9)        
remote: Compressing objects:  88% (8/9)        
remote: Compressing objects: 100% (9/9)        
remote: Compressing objects: 100% (9/9), done.        
remote: Total 12 (delta 3), reused 4 (delta 3), pack-reused 0        
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HEAD is now at 96ead91f0f GH-120804: Remove `get_child_watcher` and `set_child_watcher` from asyncio (#120818)
Switched to and reset branch 'main'

configure: WARNING: no system libmpdecimal found; falling back to bundled libmpdecimal (deprecated and scheduled for removal in Python 3.15)

make: *** [Makefile:2252: buildbottest] Error 2