Make sure a plain "tox" command works in all platforms without failures,
which facilitates making releases using a "devpi upload/test" workflow
* Fix small flakes failures
* Limit py35 testing to pytest>=2.7, as pytest<2.7 does not work on py35
* pexpect environments only work on Linux platforms and should be skipped on Windows
* Simplify AppVeyor script by running all tox environments: no need to have a build matrix
because AppVeyor does not execute builds in parallel
* Add all environments to travis.yml, obtained from "tox --listenvs"
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