Replace yield_fixture() by fixture()

`@pytest.yield_fixture` is deprecated since `pytest-3.0`, see:
https://docs.pytest.org/en/latest/yieldfixture.html

Author: atugushev

tests/helper.py

@@ -63,15 +63,15 @@ def handler(request):
     return request.param
 
 
-@pytest.yield_fixture
+@pytest.fixture
 def logger(handler):
     logger_ = logging.getLogger("test_logger")
     logger_.addHandler(handler)
     yield logger_
     logger_.removeHandler(handler)
 
 
-@pytest.yield_fixture
+@pytest.fixture
 def formatted_logger(handler):
     logger_ = logging.getLogger("formatted_test_logger")
     handler.setFormatter(logging.Formatter("%(levelname)s : %(message)s"))

tests/integration/test_extra_fields.py

@@ -47,7 +47,7 @@ def handler(request):
     return request.param
 
 
-@pytest.yield_fixture
+@pytest.fixture
 def logger(handler):
     logger = logging.getLogger("test")
     dummy_filter = DummyFilter()

tests/unit/test_handler.py

@@ -33,7 +33,7 @@ def __repr__(self):
         return "<TestClass>"
 
 
-@pytest.yield_fixture
+@pytest.fixture
 def mock_send(handler):
     try:
         with mock.patch.object(handler, "send") as mock_send: