Run Travis CI tests in OS X based environment

[webknjaz]

• PoC of running tests under MacOS
• reusable YAML anchors reduce config duplication dramatically
• completely pyenv based python installations

[webknjaz]

@jaraco I've found a working way to run tests under OS X in TravisCI. Should we run them across all envs or maybe just a few?

[jaraco]

This all seems reasonable to me. It does feel quite a bit like it's adding a good deal of complication to the config, increasing the barrier to entry for another user to work with. That said, it's not in the main code, so that's not a serious concern. If that complication is necessary, it's nice that you're using less common YAML constructs to employ reuse and avoid repetition.

I wonder how much value we get from running the tests on all Python versions on macOS (for each push/PR). I'd be content with just Python latest and (maybe) Python 2.7 or macOS to capture possible nuances with Mac, but otherwise rely on Linux for the full suite of supported versions. But you've done the hard work of getting the matrix running, so it's probably reasonable to keep it. I don't think cheroot will see so much action that the extra load on Travis is a concern.

Thanks!

[jaraco]

I generally try to avoid commented code. I suggest either marking this test as an allowed failure (if you intend for it to work in the near future) or just remove it (and let the source code history remember it in case we choose to revive it).

[jaraco]

When you merge this PR, please also cut a new release. Thanks.

[webknjaz]

Actually I was worried about the feedback time period from CI: it largely increases waiting time of "Okay, tests are green" or "Oops.. Smth is wrong with your changes in specific env". That's my primary motivation of asking about leaving just a few OS X tests.

Regarding "the latest 2.7" (etc.) I've tried that, but pyenv has it just for a few envs, so I wanted to be consistent across config, given that I follow declarative manner of specifying versions (although I could actually script version lookup and match via pyenv install --list).