Okdata-sdk

* Add function for listing webhook tokens for a dataset.

* Rename event stream methods:
* `enable_subscribable` > `enable_subscription`
* `disable_subscribable` > `disable_subscription`
* `add_sink` > `enable_sink`
* `remove_sink` > `disable_sink`

* Add event stream API support.

* Use the same default value for `cacheCredentials` in both development and
production environments.
* Update Makefile for better Python 3 compatibility.

* Add function for downloading data.
* Call `raise_for_status()` when not using SDK class.
* Reintroduce Python 3.6 support.

* Check if `refresh token` is expired before calling
`KeycloakOpenID.refresh_token()`.