pytest>=3.4 requirement is not being resolved by pip

[effigies]

We require pytest>=3.0 and pytest-xdist. The latest pytest-xdist fails for pytest<3.4, but pip does not see it as a requirement, and so we get runtime failures for tests on Travis (which comes with pytest 3.3.0 installed).

[effigies]

For reference, here's a Travis log: https://travis-ci.org/nipy/nipype/jobs/408947071

[nicoddemus]

Thanks @effigies for writing,

Our install_requires seems correct:

pytest-xdist/setup.py

Line 3 in b814db4

install_requires = ['execnet>=1.1', 'pytest>=3.4', 'pytest-forked']

Unfortunately pips dependency solver is known to have some problems. Not sure what we can do on our side to fix this.

[nicoddemus]

Note to self: do not change version requirements in bug fix versions btw.

[effigies]

Hmm. Okay. It looks like we're not upgrading our pip to the latest, so hopefully that resolves it. If not, we can just update our minimum version of pytest. 3.4 seems old enough.