"If use _testcapi the tests should be decorated with cpython_only."

at first, I thought so too, but then I searched for 'cpython_only' in
Lib/ctypes/test, and found nothing. then I searched for '_testcapi' there,
and found a top level 'import _testcapi' in
Lib/ctypes/test/test_structures.py, and a test using _testcapi.INT_MAX.

so I assumed that test_ctypes itself is cpython_only.

should test_structures.py be changed, then?