There were two problems which made it extremely annoying to terminate update-wikidata related to the retry logic:

1. retry_with_backoff did not respect ctrl_c_pressed and continued to retry although a termination was requested
2. KeyboardInterrupt is only raised after time.sleep in retry_with_backoff, which can take substantial time

Moved retry_with_backoff into the class and respect ctrl_c_pressed. Use Threading.Event for interruptible waiting. This also made the waiting in another place nicer.