Use pytest instead of py.test

[sliverc]

py.test is the old way of running pytest.

Also remove DJANGO_SETTINGS_MODULE env when running tests. This is already defined in pytest.ini and py.test will take it from as it should also take parameters we might add in pytest.ini in the future.

I have overlooked this change in #498

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• changelog entry added to CHANGELOG.md
• author name in AUTHORS

[n2ygk]

I was not aware pytest.ini set DJANGO_SETTINGS_MODULE. Thanks for that.
Please resolve the flake8 travis failure (caused by merging #501?).

[sliverc]

@n2ygk
Rebased with master and CI runs through now

[n2ygk]

@sliverc It's out of date again since you merged another PR. Should I just go ahead and click on "Update branch" and save you the time in the future?

[sliverc]

@n2ygk
I have pressed update now. But feel free to update in the future. Or you can also approve the changes with the comment to update before merging. This way I can update and can merge in one go. Whatever you prefer.