travis CI pytest deprecation warning

[mikofski]

If you check the latest build it says:

=============================== warnings summary ===============================
None
  [pytest] section in setup.cfg files is deprecated, use [tool:pytest] instead.
-- Docs: http://doc.pytest.org/en/latest/warnings.html

The requirements.txt lists pytest>=2.8.5; pytest-3.3.0 is currently always installed by default in Travis, so when they upgrade to pytest-4.x.x, then the warning will become an error.

Also just noticed that Travis is only testing python-2.7, but that's a separate issue, maybe related to #12 and setting up Azure pipelines to build/test python-3