    async def check_hub_version(self):
        """Test a connection to my Hub

        - exit if I can't connect at all
        - check version and warn on sufficient mismatch
        """
        client = self.hub_http_client
        RETRIES = 5
        for i in range(1, RETRIES + 1):
            try:
                resp = await client.fetch(self.hub_api_url)
            except Exception:
                self.log.exception(
                    "Failed to connect to my Hub at %s (attempt %i/%i). Is it running?",
                    self.hub_api_url,
                    i,
                    RETRIES,
                )
                await asyncio.sleep(min(2**i, 16))
            else:
                break
        else:
            self.exit(1)

        hub_version = resp.headers.get('X-JupyterHub-Version')
        _check_version(hub_version, __version__, self.log)
Test a connection to my Hub

- exit if I can't connect at all
- check version and warn on sufficient mismatch
jupyterhub/singleuser/mixins.py