Switch to pytest runner

[mikemhenry]

As a first pass in improving our testing, we are going to switch to using pytest to run tests. Because of the compatibility between pytest and nose we shouldn't need to do anything to the old tests, but we should start writing new tests using pytest features.

[mikemhenry]

Note: I biased this PR off of #761 since it will be merged soon and fixes some issues with the testing environment.

[codecov]

Codecov Report

Merging #763 (fc1e0d5) into master (317a3bb) will decrease coverage by 8.97%.
The diff coverage is n/a.

[jaimergp]

Awesome, works right away!

Minor things:

• This kind of warning is probably due to a docstring containing a literal \P (as in C:\Program Files), or maybe some kind of latex formula. They will go away as soon as you turn those docstrings into raw-docstrings (r"""some text""")

By the way, this might be a good moment to enable tests for the examples/ directory in a separate GHA step, if that's not planned (also in a different PR if you wish).

[mikemhenry]

Thanks! I will go through and fix those warnings on this PR since they really clog up the CI results. I've made a plan here to add the examples to the CI here #766