Add runtime checking to create_server to verify that startup has been run (#2328)

examples/run_async.py

@@ -1,7 +1,5 @@
 import asyncio
 
-from signal import SIGINT, signal
-
 import uvloop
 
 from sanic import Sanic, response
@@ -15,17 +13,18 @@ async def test(request):
     return response.json({"answer": "42"})
 
 
-asyncio.set_event_loop(uvloop.new_event_loop())
-server = app.create_server(
-    host="0.0.0.0", port=8000, return_asyncio_server=True
-)
-loop = asyncio.get_event_loop()
-task = asyncio.ensure_future(server)
-server = loop.run_until_complete(task)
-loop.run_until_complete(server.startup())
-signal(SIGINT, lambda s, f: loop.stop())
+async def main():
+    server = await app.create_server(
+        port=8000, host="0.0.0.0", return_asyncio_server=True
+    )
 
-try:
-    loop.run_forever()
-finally:
-    loop.stop()
+    if server is None:
+        return
+
+    await server.startup()
+    await server.serve_forever()
+
+
+if __name__ == "__main__":
+    asyncio.set_event_loop(uvloop.new_event_loop())
+    asyncio.run(main())