Use pytest.fixture rather than deprecated yield_fixture

[jab]

Just took this project for a spin for the first time and noticed this while reading the code.

Quoting https://docs.pytest.org/en/stable/yieldfixture.html:

Since pytest-3.0, fixtures using the normal fixture decorator can use a yield statement to provide fixture values and execute teardown code, exactly like yield_fixture in previous versions.

Marking functions as yield_fixture is still supported, but deprecated and should not be used in new code.

Hope this helps, and thanks for maintaining pytest-flask!

[northernSage]

By the way, thanks a lot for your contribution @jab 👍🏻

[jab]

My pleasure, and thank you for merging it! 😊