[tests] Fixed deprecated use of yield_fixture in test.

Signed-off-by: Michael Seifert <m.seifert@digitalernachschub.de>

tests/test_subprocess.py

@@ -8,7 +8,7 @@
     # The default asyncio event loop implementation on Windows does not
     # support subprocesses. Subprocesses are available for Windows if a
     # ProactorEventLoop is used.
-    @pytest.yield_fixture()
+    @pytest.fixture()
     def event_loop():
         loop = asyncio.ProactorEventLoop()
         yield loop