don't use deprecated pytest.yield_fixture in readme

[Mattwmaster58]

See https://docs.pytest.org/en/latest/yieldfixture.html:

Important

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.

[simonfagerholm]

Duplicate of #138

[Tinche]

Merged in the other one, thanks!