travis pytest version conflict

[kumom]

Describe the bug
Travis build failed with the hint
pkg_resources.VersionConflict: (pytest 3.3.0 (/home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages), Requirement.parse('pytest>=3.6')).

To Reproduce
I used my branch position_agent and the original setup.py to build.