Try out the flake8-pytest linter

[abadger]

@webknjaz suggested we try out the following linter for giving our tests a consistent style:

m-burst/flake8-pytest-style#23

I know how tests tend to get very ad hoc and inscrutable as they grow over time so it would be good to try that out and see if it fits with how we want to write tests and is helpful for making tests which are more readable later.

[webknjaz]

FTR here's how I configure it (because I prefer things to be slightly different from defaults):

# flake8-pytest-style
# PT001:
pytest-fixture-no-parentheses = true
# PT006:
pytest-parametrize-names-type = tuple
# PT007:
pytest-parametrize-values-type = tuple
pytest-parametrize-values-row-type = tuple