add comment explaining the keepalive fixture

Author: amirreza8002

tests/tests_async/conftest.py

@@ -6,6 +6,7 @@ def anyio_backend():
     return "asyncio"
 
 
+# this keeps the event loop open for the entire test suite
 @pytest.fixture(scope="session", autouse=True)
 async def keepalive(anyio_backend):
     pass