Error: Call to a member function request() on string in Mailchimp\MailchimpLists->getMemberInfo()

After reading the docs for mailchimp at https://github.com/thinkshout/mailchimp-api-php, it turns out the way you pass in the api_key has changed in version 3.0. Now you pass in an authentication array to get the api_class variable, then pass that into the MailchimpLists.

      $config = Drupal::config('mailchimp.settings');
      $api_key = $config->get("api_key");

      // Instantiate MailchimpApiInterface
      $authentication_settings = [
        'api_key' => $api_key,
        'api_user' => 'api_key',
      ];
      
      // Use Mailchimp class for api_key.
      $api_class = new Mailchimp($authentication_settings);

      $mailChimpLists = new MailchimpLists($api_class);
      $listID = '123456789';
      $email = 'myemail@gmail.com';
      $member = $mailChimpLists->getMemberInfo($listID, $email);