I don't think that it makes sense to call _PyCapsule_SetTraverse() with NULL traverse function. If a traverse function is defined, it would look suspicious that there is no associated clear function. So yeah, I think that it's a better API to require both callbacks to be non-NULL.

Thanks for your post-merge review :-)

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot aarch64 RHEL8 LTO 3.x has failed when building commit a35d48d.

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/338/builds/4868) and take a look at the build logs.
4. Check if the failure is related to this commit (a35d48d) 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/338/builds/4868

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

== Tests result: ENV CHANGED ==

441 tests OK.

10 slowest tests:

• test_concurrent_futures: 2 min 21 sec
• test_gdb: 2 min 21 sec
• test_signal: 1 min 12 sec
• test.test_multiprocessing_spawn.test_processes: 40.9 sec
• test_io: 38.3 sec
• test_socket: 35.3 sec
• test.test_multiprocessing_forkserver.test_processes: 34.1 sec
• test_math: 34.0 sec
• test_imaplib: 32.2 sec
• test_subprocess: 28.4 sec

1 test altered the execution environment:
test_interpreters

14 tests skipped:
test.test_asyncio.test_windows_events
test.test_asyncio.test_windows_utils test_devpoll test_ioctl
test_kqueue test_launcher test_startfile test_tkinter test_ttk
test_winconsoleio test_winreg test_winsound test_wmi
test_zipfile64

Total duration: 2 min 30 sec

Traceback (most recent call last):
  File "<frozen importlib._bootstrap>", line 1266, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1237, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 841, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 1000, in exec_module
  File "<frozen importlib._bootstrap_external>", line 1096, in get_code
  File "<frozen importlib._bootstrap_external>", line 1195, in get_data
TypeError: descriptor 'close' for '_io.BufferedReader' objects doesn't apply to a '_io.FileIO' object


Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto/build/Lib/threading.py", line 1059, in _bootstrap_inner
    self.run()
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto/build/Lib/threading.py", line 996, in run
    self._target(*self._args, **self._kwargs)
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto/build/Lib/test/test_interpreters.py", line 481, in task
    interp = interpreters.create()
             ^^^^^^^^^^^^^^^^^^^^^
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto/build/Lib/test/support/interpreters.py", line 25, in create
    id = _interpreters.create(isolated=isolated)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
RuntimeError: interpreter creation failed
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