Fatal error, need to update EntityInterface entityType to be static

This is the same as https://github.com/backdrop-contrib/flag/issues/14

Core had an API change. Need an update here like this:

```php
          /**
		   * Implements EntityInterface::entityType().
		   *
		   * @since 1.13.0 This method is now static.
		   */
		  public static function entityType() {
		    return 'entity_test';
		  }
```