[python/asyncio] explicitly close client session via async context manager (#5621)

samples/client/petstore/python-asyncio/petstore_api/api_client.py

@@ -81,13 +81,14 @@ class ApiClient(object):
         self.user_agent = 'OpenAPI-Generator/1.0.0/python'
         self.client_side_validation = configuration.client_side_validation
 
-    def __enter__(self):
+    async def __aenter__(self):
         return self
 
-    def __exit__(self, exc_type, exc_value, traceback):
-        self.close()
+    async def __aexit__(self, exc_type, exc_value, traceback):
+        await self.close()
 
-    def close(self):
+    async def close(self):
+        await self.rest_client.close()
         if self._pool:
             self._pool.close()
             self._pool.join()

samples/client/petstore/python-asyncio/petstore_api/rest.py

@@ -18,7 +18,6 @@ import ssl
 
 import aiohttp
 import certifi
-import asyncio
 # python 2 and python 3 compatibility library
 from six.moves.urllib.parse import urlencode
 
@@ -85,8 +84,8 @@ class RESTClientObject(object):
                 connector=connector
             )
 
-    def __del__(self):
-        asyncio.ensure_future(self.pool_manager.close())
+    async def close(self):
+        await self.pool_manager.close()
 
     async def request(self, method, url, query_params=None, headers=None,
                       body=None, post_params=None, _preload_content=True,