Use `setup.cfg` file for flake8

I used the configuration from django/setup.cfg.

- fixed extra blank lines
- fixed some minor issues with the `_` variable in `test_user`
- added to excluded F841 for unused variables, this needs more work
Fix F841 errors of unused local variables
Merge pull request #34 from benjaminabel/flake8-in-setup

Use `setup.cfg` file for flake8