Add flake8 without RecursionError

[cclauss]

Description

Another approach to @leezu #219 that is based on some of the ideas in the review of #213 that were lost when that PR was simplified. Our goal is to add flake8 tests while avoiding a RecursionError. We do this by creating three different steps in Makefile that can be run separately or together via a unified make lint command. This allows contributors to run these tests separately or together.

Checklist

Essentials

• Changes are complete (i.e. I finished coding on this PR)
• All changes have are test coverage
• Code is well-documented

Changes

• Add make flake8, make restruc, and make lint to Makefile

Comments

[leezu]

Thanks @cclauss . I have pushed your commit in #219 , as the CI doesn't allow you updating the Jenkinsfile for security reasons. But the RecursionError remains. I believe it is an issue with the conda setup, though I am not yet sure what exactly.
Sorry for not answering to your cleaned up proposal in #213 yet (ie. what you have put in to a commit here now). For #219 it was not intended to merge yet but to debug why the CI fails. I mean, your contribution in #213 was very welcome and valued, just as I'm still debugging the conda setup, not yet included.

[cclauss]

OK.... Closing.

[leezu]

@cclauss Thanks for your contribution. I merged your commit together with a fix for the RecursionError. The issue was that flake8 tried to check all files in the local conda environment, which is in ./conda in the checked out directory.

[leezu]

Also see PyCQA/pyflakes#347