Fix async_timeout deprecation warning. (#471)

Author: yuxincs

src/adguardhome/adguardhome.py

@@ -132,7 +132,7 @@ async def request(
             self._close_session = True
 
         try:
-            with async_timeout.timeout(self.request_timeout):
+            async with async_timeout.timeout(self.request_timeout):
                 response = await self._session.request(
                     method,
                     url,