The new CLI was moved over to a separate package, plann - so the calendar-cli is in maintenance mode.
Since the caldav library now ships with dependencies on niquests and the fork urllib3.future, things break when doing an import urllib3 . The only thing urllib3 was used for, was to mute security warnings when a user explicitly asks for connection with a server with invalid certificate. This most likely doesn't work more anyway, so it was better to remove the urllib3 import than to add urllib3 as a dependency.
See also #113