Update this project to use the Sync API v9

[drmfinlay]

Every few years I come back to this and find it broken by deprecation. The code now needs to be updated to use either the Sync API v9, which doesn't have an official Python package, or the new REST API. The former is preferred, since it is what this project used originally.

Shouldn't be too difficult. There are only some minor differences and I wish to maintain this project.

Todoist's migration guide for v8 to v9 is available here: https://developer.todoist.com/sync/v9/#migrating-from-v8.

[drmfinlay]

Okay, everything except the optional inclusion of archived sections and projects is fixed. Todoist have changed this.