The bug only occurs in Python 3.5, right?

> Sometimes the test suite freezes in test_interrupted_write_retry_text (test.test_io.CSignalsTest).

How do you run the test suite? Is your system heavily loaded? Is it "fast"?

This is an obvious race condition in the test if SIGARLM is send before write() is called:

            signal.alarm(1)
            # Expected behaviour:
            # - first raw write() is partial (because of the limited pipe buffer
            #   and the first alarm)
            # - second raw write() returns EINTR (because of the second alarm)
            # - subsequent write()s are successful (either partial or complete)
            self.assertEqual(N, wio.write(item * N))

1 second should be enough :-)

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Or it is maybe a regression caused by the changeset 5b63010be19e of issue #23285 (PEP 475).