[Twisted-Python] very weird 16.4 upgrade breakage

[Glyph Lefkowitz]

On Mimic, the requires.io PR that upgraded us to Twisted 16.4 broke the build.

It did so in a really weird way: https://travis-ci.org/rackerlabs/mimic/jobs/156439316

Specifically, our mac app CI tests failed like so:

  File "pkg_resources/extern/__init__.pyc", line 61, in load_module
    
exceptions.ImportError: The 'appdirs' package is required; normally this is bundled with this package so if you get this warning, consult the packager of your distribution.

Any idea why 'appdirs' might suddenly become a hard dependency of pkg_resources, but only in the py2app bundle?  Did something change about the way we referenced dependencies?

-glyph
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