Fix installation with Python 3.7. Fixes #1414.

Fix deprecated use of aiohttp.Timeout. Fixes #1296.
Use "async with" with aiohttp.ClientSession().
Make sure websocket connections are properly closed, see https://docs.aiohttp.org/en/stable/web_advanced.html#graceful-shutdown
Finish to drop Python 3.4.

gns3server/handlers/api/controller/notification_handler.py

@@ -69,14 +69,17 @@ class NotificationHandler:
         ws = aiohttp.web.WebSocketResponse()
         await ws.prepare(request)
 
+        request.app['websockets'].add(ws)
         asyncio.ensure_future(process_websocket(ws))
         with controller.notification.controller_queue() as queue:
             while True:
                 try:
                     notification = await queue.get_json(5)
+                    if ws.closed:
+                        break
+                    await ws.send_str(notification)
                 except asyncio.futures.CancelledError:
                     break
-                if ws.closed:
-                    break
-                ws.send_str(notification)
+                finally:
+                    request.app['websockets'].discard(ws)
         return ws

gns3server/handlers/api/controller/project_handler.py

@@ -261,17 +261,19 @@ class ProjectHandler:
         ws = aiohttp.web.WebSocketResponse()
         await ws.prepare(request)
 
+        request.app['websockets'].add(ws)
         asyncio.ensure_future(process_websocket(ws))
-
         with controller.notification.project_queue(project) as queue:
             while True:
                 try:
                     notification = await queue.get_json(5)
-                except asyncio.futures.CancelledError as e:
+                    if ws.closed:
+                        break
+                    await ws.send_str(notification)
+                except asyncio.futures.CancelledError:
                     break
-                if ws.closed:
-                    break
-                ws.send_str(notification)
+                finally:
+                    request.app['websockets'].discard(ws)
 
         if project.auto_close:
             # To avoid trouble with client connecting disconnecting we sleep few seconds before checking