Run flake8 static checks separately from test suite

[takluyver]

pytest-flake8 is a compatibility headache - see tholo/pytest-flake8#81 tholo/pytest-flake8#83 & tholo/pytest-flake8#87 . The advantages of running it inside pytest seem small at best, so this gets rid of the pytest integration in favour of running flake8 directly. In the CI, isort & flake8 are run as part of a 'static-checks' job.

[takluyver]

To be clear, I'm not trying to criticise the developers of pytest-flake8 - I think compatibility headaches are probably unavoidable. Flake8 is pretty clear that it's an application, not a library, and plugin APIs (it's a pytest plugin) are always difficult, because eventually the application developers are going to have a choice between slightly breaking a plugin API and holding back the application development.

I'm going to merge this since one of those compatibility issues is currently causing the CI to fail.