fix async test

Author: mabdinur

ddtrace/contrib/internal/aiohttp/middlewares.py

@@ -49,7 +49,7 @@ async def attach_context(request):
         ) as ctx:
             req_span = ctx.span
 
-            ctx.set_item("req_span", req_span)
+            ctx.set_item("req_span")
             core.dispatch("web.request.start", (ctx, config.aiohttp))
 
             # attach the context and the root span to the request; the Context