Force client class to be async when kernel manager is MultiKernelManager

Author: davidbrochart

nbclient/client.py

@@ -388,6 +388,9 @@ async def async_start_new_kernel_client(self, **kwargs):
             except AttributeError:
                 raise AttributeError('self.km={} has no client() or get_kernel() method, '
                                      'what is this?'.format(self.km))
+            # since self.km is a MultiKernelManager, it might not be async
+            # so let's ensure the client is async
+            km.client_class = 'jupyter_client.asynchronous.AsyncKernelClient'
 
         self.kc = km.client()
         await ensure_async(self.kc.start_channels())