Switch tests to pytest #756

UDPATE: Marking this as a "good first issue" for someone starting to get familiar with OPS or with Python/git/GitHub. The best way to do this is to pick one file (or even part of one file) and do that change the tests from using nose's assert_equals(a, b) functions to using the more native assert a == b used in Pytest. A PR that did that for even a few classes would be a very welcome contribution.

We should switch from nosetests to pytest. Nose has been dead (no longer under active development) for a while. It seems like pytest is what "all the cool kids" are using these days.

This will take a couple PRs to implement, which is why I'm putting this in an issue. The main steps I see, in order to get this working reasonably well:

• Fix namings so that they work with pytest (current tests should also run under pytest) Pytest compatibility #758
• Switch to using pytest on Travis Switch to pytest on Travis #808
• Remove nose-specific imports; replace with pytest-native approaches

The last step might be completed in several steps, since we can develop new tests with pytest-native approaches while still using the old nose imports within pytest.

At some point in the future, it would be really nice to also make some of the tests run in a more pytest-native way (there are additional fixtures and things like parametrization that can be done in pytest that aren't available in nose; we accomplish the same in our tests by writing more code). That overhaul may happen piece by piece after the above tasks are complete -- at least, I don't have time to sit down and do a full overhaul of the test suite.