Flaky test: AttachContainerTest.test_attach_no_stream

[thaJeztah]

We've seen this test fail in moby CI (disabled it at some point moby/moby#39848, but it looks to be still flaky after re-enabling it recently when we updated to the latest docker-py version moby/moby#40467)

[2020-02-10T23:40:44.429Z] =================================== FAILURES ===================================
[2020-02-10T23:40:44.429Z] __________________ AttachContainerTest.test_attach_no_stream ___________________
[2020-02-10T23:40:44.429Z] tests/integration/api_container_test.py:1250: in test_attach_no_stream
[2020-02-10T23:40:44.429Z]     assert output == 'hello\n'.encode(encoding='ascii')
[2020-02-10T23:40:44.429Z] E   AssertionError: assert b'' == b'hello\n'
[2020-02-10T23:40:44.429Z] E     Right contains more items, first extra item: 104
[2020-02-10T23:40:44.429Z] E     Use -v to get the full diff
[2020-02-10T23:40:44.429Z] ------- generated xml file: /src/bundles/test-docker-py/junit-report.xml -------

Perhaps this needs a change similar to ef04355 (#2307)