check for already set asyncio event loop policy (#1637)

* check for already set asyncio event loop policy

* fix linting warning

sanic/server.py

@@ -30,6 +30,7 @@ from sanic.response import HTTPResponse
 try:
     import uvloop
 
+    if not isinstance(asyncio.get_event_loop_policy(), uvloop.EventLoopPolicy):
         asyncio.set_event_loop_policy(uvloop.EventLoopPolicy())
 except ImportError:
     pass