Use pytest.fixture rather than deprecated yield_fixture (#131)

Author: jab

pytest_flask/fixtures.py

@@ -30,7 +30,7 @@ def deprecated_call(*args, **kwargs):
     return decorator
 
 
-@pytest.yield_fixture
+@pytest.fixture
 def client(app):
     """A Flask test client. An instance of :class:`flask.testing.TestClient`
     by default.