Hunting reference leaks with "./python -m test -R 3:3 test_datetime" crashs since the commit 04af5b1ba9eb546a29735fac6cb5298159069b53.

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vstinner@apu$ ./python -m test -R 3:3 test_datetime 
Run tests sequentially
0:00:00 load avg: 1.05 [1/1] test_datetime
beginning 6 repetitions
123456
Fatal Python error: Segmentation fault

Current thread 0x00007ff71fe33040 (most recent call first):
  File "/home/vstinner/prog/python/master/Lib/test/datetimetester.py", line 5464 in test_utc_capi
  (...)
Segmentation fault (core dumped)
***

Shorter example without -R, just run the same test twice:
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vstinner@apu$ ./python -m test -v test_datetime test_datetime -m test_utc_capi
== CPython 3.7.0a4+ (heads/master:cab0b2b053, Jan 25 2018, 09:06:01) [GCC 7.2.1 20170915 (Red Hat 7.2.1-2)]
== Linux-4.14.13-300.fc27.x86_64-x86_64-with-fedora-27-Twenty_Seven little-endian
== cwd: /home/vstinner/prog/python/master/build/test_python_26308
== CPU count: 8
== encodings: locale=UTF-8, FS=utf-8
Run tests sequentially
0:00:00 load avg: 1.26 [1/2] test_datetime
test_utc_capi (test.datetimetester.CapiTest_Pure) ... skipped 'Not relevant in pure Python'
test_utc_capi (test.datetimetester.CapiTest_Fast) ... ok

----------------------------------------------------------------------
Ran 2 tests in 0.001s

OK (skipped=1)
0:00:00 load avg: 1.26 [2/2] test_datetime
test_utc_capi (test.datetimetester.CapiTest_Pure) ... skipped 'Not relevant in pure Python'
test_utc_capi (test.datetimetester.CapiTest_Fast) ... Fatal Python error: Segmentation fault

Current thread 0x00007f9836b31040 (most recent call first):
  File "/home/vstinner/prog/python/master/Lib/test/datetimetester.py", line 5464 in test_utc_capi
  (...)
Segmentation fault (core dumped)
***