Oh... test_threaded_import started to crash on Python 2.7:
$ ./python -m test -F -v test_threaded_import
(...)
0:00:00 load avg: 1.06 [ 3] test_threaded_import
Trying 20 threads ... OK.
Trying 50 threads ... OK.
Trying 20 threads ... OK.
Segmentation fault (core dumped)
The problem is that PyMem_Malloc() is not thread safe when Python is compiled in debug mode! In release mode, it's safe because PyMem_Malloc() is a thin wrapper to malloc(). But in debug mode, PyMem_Malloc() uses the debug hooks and... pymalloc allocator which is not thread-safe!