Update manager.py

fix kernel can only resstart once  issue

jupyter_client/manager.py

@@ -309,6 +309,7 @@ async def _async_post_start_kernel(self, **kw) -> None:
         `**kw` : optional
              keyword arguments that were used in the kernel process's launch.
         """
+        self.shutting_down = False
         self.start_restarter()
         self._connect_control_socket()
         assert self.provisioner is not None