In Python 3.9, test_pha_required_nocert() looks more strict, it requires read() to fail with the 'tlsv13 alert certificate required' error message:

        # receive alert
        with self.assertRaisesRegex(
                ssl.SSLError,
                'tlsv13 alert certificate required'):
            s.recv(1024)

In the main branch, it tolerates "EOF occurred" error:

    # test sometimes fails with EOF error. Test passes as long as
    # server aborts connection with an error.
    with self.assertRaisesRegex(
        ssl.SSLError,
        '(certificate required|EOF occurred)'
    ):
        # receive CertificateRequest
        data = s.recv(1024)
        self.assertEqual(data, b'OK\n')

        # send empty Certificate + Finish
        s.write(b'HASCERT')

        # receive alert
        s.recv(1024)