Replace @asyncio.coroutine decorator with async def

In Python 3.11 @asyncio.coroutine decorator was removed and it should
be replaced with async def call.

Fixes: aio-libs#280

tests/unit/test_cors_config.py

@@ -29,8 +29,7 @@ async def _handler(request):
 
 class _View(web.View, CorsViewMixin):
 
-    @asyncio.coroutine
-    def get(self):
+    async def get(self):
         return web.Response(text="Done")