Screenshots won't work with pytest 5

In _browser_screenshot_session, you have the following code:
```

    fixture_values = (
        # pytest 3
        getattr(request, '_fixture_values', {}) or
        # pytest 2
        getattr(request, '_funcargs', {})
    )

    for name, value in fixture_values.items():
```

None of these getters are set in pytest 5.