pytest version 3.10 Fails on python2.7

[henrykironde]

python2.7.14/lib/python2.7/site-packages/_pytest/stepwise.py", line 102, in pytest_sessionfinish
self.config.cache.set("cache/stepwise", [])
AttributeError: 'Config' object has no attribute 'cache'

ref: https://travis-ci.org/weecology/retriever/jobs/451239186

checklist:

• Include a detailed description of the bug or suggestion
• pip list of the virtual environment you are using
• pytest and operating system versions
• Minimal example if possible

[blueyed]

AttributeError: 'Config' object has no attribute 'cache'

Appears to be a duplicate of #4304.