pytest-runner should not be a hard-requirement in setup.py

[shoyer]

#2573 (by @horta) added pytest-runner to setup_requires in our setup.py file.

This had the unintentional consequence of making pytest-runner a requirement for installing xarray. This is causing our conda-forge deployment to fail: conda-forge/xarray-feedstock#42

We really want a "conditional requirement" only for testing as described in the pytest-runner docs:
https://pypi.org/project/pytest-runner/

My plan is to do the conditional requirement, and back-port the commit to make a new 0.11.2 release.