implementing the revert suggested by @shin- ages ago

[abrugh]

Fixes the issue described, tested and reproduced in docker/compose#6018

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "6018" git@github.com:abrugh/docker-py.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[ulyssessouza]

This PR is not passing due to an already solved problem.
Please rebase your PR with master

[abrugh]

Rebased, still failing AttachContainerTest::test_attach_stream_and_cancel but only 2 out of 4 times....

[shin-]

Looks like it's failing pretty consistently on Python 2.7 (if you look at the test matrix here). To me that suggests that that condition does something helpful, and the fix is less trivial than it appears.