Support pytest 4

[hroncok]

Fixes https://github.com/kennethreitz/requests/issues/5048

See https://docs.pytest.org/en/latest/deprecations.html#marks-in-pytest-mark-parametrize

[hroncok]

Not sure about the CI here, it seems to be failing on master as well.
I've also tried to update the Pipfile.lock, but I have no idea how to do it properly without regenerating the entire Pipfile.lock. Tried some magic.

[luckydenis]

Fix some issues to improve adherence to best practices

Facepalm, not the and not there wrote. Sorry.

[hroncok]

I've dropped the pipfile changes, they conflicted.