gh-108240: Add pycore_capsule.h internal header file

[vstinner]

Move _PyCapsule_SetTraverse() definition to a new internal header file: pycore_capsule.h.

• Issue: _socket extension C capsule cannot be visited by the GC and so _socket.socket type may stay alive longer than expected #108240

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Windows10 3.x has failed when building commit 39506ee.

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/146/builds/6146) and take a look at the build logs.
4. Check if the failure is related to this commit (39506ee) 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/146/builds/6146

Failed tests:

• test.test_concurrent_futures.test_deadlock

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

==

Click to see traceback logs

Traceback (most recent call last): File "D:\buildarea\3.x.bolen-windows10\build\Lib\threading.py", line 1059, in _bootstrap_innerself.run() File "D:\buildarea\3.x.bolen-windows10\build\Lib\concurrent\futures\process.py", line 344, in runself.terminate_broken(cause) File "D:\buildarea\3.x.bolen-windows10\build\Lib\concurrent\futures\process.py", line 492, in terminate_broken work_item.future.set_exception(bpe) File "D:\buildarea\3.x.bolen-windows10\build\Lib\concurrent\futures\_base.py", line 559, in set_exceptionraise InvalidStateError('{}: {!r}'.format(self._state, self)) concurrent.futures._base.InvalidStateError: CANCELLED: <Future at 0x289f4cbca60 state=cancelled> Warning -- threading_cleanup() failed to cleanup 1 threads (count: 1, dangling: 2) Warning -- Dangling thread: <Thread(QueueFeederThread, started daemon 8632)> Warning -- Dangling thread: <_MainThread(MainThread, started 1476)>