Retain an assertion and update comments

jupyterhub/tests/conftest.py

@@ -55,10 +55,16 @@ _db = None
 
 
 def pytest_collection_modifyitems(items):
-    """add asyncio marker to all async tests"""
+    """This function is automatically run by pytest passing all collected test
+    functions.
+
+    We use it to add asyncio marker to all async tests and assert we don't use
+    test functions that are async generators which wouldn't make sense.
+    """
     for item in items:
         if inspect.iscoroutinefunction(item.obj):
             item.add_marker('asyncio')
+        assert not inspect.isasyncgenfunction(item.obj)
 
 
 @fixture(scope='module')