test compat with pytest 4.0

[wholmgren]

• Closes fix tests #623
• I am familiar with the contributing guidelines.
• Fully tested. Added and/or modified tests to ensure correct behavior for all reasonable inputs. Tests (usually) must pass on the TravisCI and Appveyor testing services.
• Adds description and name entries in the appropriate docs/sphinx/source/whatsnew file for all changes.
• Code quality and style is sufficient. Passes LGTM and SticklerCI checks.
• New code is fully documented. Includes sphinx/numpydoc compliant docstrings and comments in the code where necessary.
• Pull request is nearly complete and ready for detailed review.

passes locally on python 3.6 with numba and pytest 4.0. I'll try python 3.7 + numba next.

[wholmgren]

All tests pass, including the python 3.7 + numba test. I don't know if it's coincidental or not.

@cwhanse ready to merge?

[wholmgren]

including the python 3.7 + numba test. I don't know if it's coincidental or not.

After merging, running tests on master still errors when trying to import spa.py, so I guess the pass was coincidental. The failing build on master used python 3.7.1 whereas the passing build in the PR used python 3.7.0. Maybe continuum needs to rebuild numba with 3.7.1? Let's wait a little longer to see if this goes away on it's own.