Fix unclosed aiohttp.ClientSession during __del__

Author: vdboor

Diff for: src/zeep/asyncio/transport.py

@@ -46,7 +46,9 @@ def __init__(self, loop, cache=None, timeout=300, operation_timeout=None,
 
     def __del__(self):
         if self._close_session:
-            self.session.close()
+            # aiohttp.ClientSession.close() is async,
+            # call the underlying sync function instead.
+            self.session.connector.close()
 
     def _load_remote_data(self, url):
         result = None