is not JSON serializable error. According to API documentation post request needs file object. That means "json.dumps" is not needed [here](https://github.com/retailcrm/api-client-python/blob/27ff0849eca746be250d35e21b304f5fb948062e/retailcrm/versions/v5.py#L745). I've made [fork](https://github.com/Pluser761/api-client-python), to fix that ([here it is](https://github.com/Pluser761/api-client-python/blob/95bf0b143a9933ef17d0cb2f18630111c310500d/retailcrm/versions/v5.py#L745)), but could not make pull request. Merge that please.