pytest integration plan

[thehrh]

I decided to open this issue as a place to document any future discussion about this topic. The PISA guide on general conventions has been stating for a long time that the plan is to integrate pytest, I presume to auto-detect, run, and report on the results of unit tests.

Installing pytest has been part of several workflows since https://github.com/icecube/pisa/pull/701, but apparently only because of the existence of two lines in two unit tests (binning.test_MultiDimBinning() and pisa_tests.test_kde_stage) that are relying on pytest.raises() as a context manager.

https://github.com/icecube/pisa/pull/580 looks as if it was meant as a custom implementation of some pytest-like functionality, avoiding it as external dependency, but I'm not sure whether this interpretation is correct. The merged branch was even called "pytest".