Sorting behaviour seemingly not stable across Python runtimes.

[amcgregor]

As can be seen in this Travis-CI test run the isort checks are failing only under Pypy runtimes, despite known_first_party being declared. What have I missed or am doing wrong?

[cs01]

We are running into this as well the Travis build for wntrblm/nox#242, even after pinning the isort version. We had to turn off the call to isort in our Travis CI test. It's too bad because isort's functionality is really useful, IMO on par with black.

[timothycrosley]

This should be fixed for the 5.0.0 release in spirit via this ticket: #1147