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pytest-flake8 1.0.1 has a hard dependency on pytest>=3.5 [1], which
does not support python3.3 [2].  Avoid it for python3.3 and below
until python3.3 is removed [3].

Additionally, ensure we have a recent enough pip in travis ci to
handle multiple requirements entries.

[1] https://github.com/tholo/pytest-flake8/commit/25bbd3b42d3aa0962fb736202115dae9e5d2cd7c
[2] https://docs.pytest.org/en/latest/changelog.html#pytest-3-3-0-2017-11-23
[3] https://github.com/pypa/setuptools/pull/1342