old_contents: c = get_config()
  c.KernelManager.kernel_cmd = [r"%s", "{connection_file}"]
  c.Session.key = ''
  c.Session.keyfile = ''
new_contents: c = get_config()
  c.KernelManager.kernel_spec = [r"%s", "{connection_file}"]
  c.Session.key = b''
  c.Session.keyfile = ''
subject: make session key a byte string