I, and my colleages are running tests mostly on linux, since our packages usually are clean python implementation. However, for mirakuru, which we use to govern processes in tests, and by extension pytest-dbfixtures, which has predefined process fixtures for pytest, we wanted to run tests also on osx. Travis.ci for a long time had option to configure osx workers and linux, however python builds on osx are kind of broken by default.
Travis.ci has a long standing bug report for this issue, which will get 2 years old in this May. Apparently, just setting python as a language and osx among the system to test on won't work for apple's operating system.