Only use @pytest.fixture decorator once

[hroncok]

This is now required in pytest >= 3.6

See pytest-dev/pytest#3518

Fixes #49

[kevin1024]

Thanks. Does this change the api at all or is it a completely internal change? Too lazy to look up what autouse does. (On the beach in Hawaii right now!)

[hroncok]

Oh. I haven't noticed that the part is part of the plugin. This might actually make this autosued for everybody and be bad :(

[hroncok]

lastly, and please use that with care: if you define an autouse fixture in a plugin, it will be invoked for all tests in all projects where the plugin is installed.

https://docs.pytest.org/en/latest/fixture.html

So a different fix is needed.

[hroncok]

Now it doesn't change the API.