bpo-40149: Implement traverse in _abc._abc_data

[vstinner]

Implement tp_traverse and tp_clear slots in _abc._abc_data type.

https://bugs.python.org/issue40149

[corona10]

Looks good, I've checked the patch on my local machine.
If CIs are passed. It is good to go :)

[shihai1991]

This PR looks good to me, but I raised this error again after I use this PR, Lol~

$ ./python -m test -R 3:3 test_threading -m test.test_threading.SubinterpThreadingTests.test_threads_join_2
0:00:00 load avg: 0.52 Run tests sequentially
0:00:00 load avg: 0.52 [1/1] test_threading
beginning 6 repetitions
123456
......
test_threading leaked [19, 19, 19] references, sum=57
test_threading leaked [12, 12, 12] memory blocks, sum=36
test_threading failed

== Tests result: FAILURE ==

1 test failed:
    test_threading

Total duration: 773 ms
Tests result: FAILURE

[vstinner]

This PR looks good to me, but I raised this error again after I use this PR, Lol~

This change doesn't completely fix https://bugs.python.org/issue40149

See https://bugs.python.org/issue40149#msg365913 and https://bugs.python.org/issue40217

[shihai1991]

This PR looks good to me, but I raised this error again after I use this PR, Lol~

This change doesn't completely fix https://bugs.python.org/issue40149

See https://bugs.python.org/issue40149#msg365913 and https://bugs.python.org/issue40217

Got it, thanks.