Review Bot 2 was using Celery 4, which is no longer maintained, and

Review Bot 3 inherited that dependency. Between Celery 4 (which is no

longer maintained and has security problems) and 5, much has changed

when it comes to the celery worker command setup and invocation, and

this is what we were wrapping in order to start a process. This is no

longer feasible.
Now, we construct the Worker object ourselves. This is more

future-proof, though comes with a few requirements on our end (some

patching functions we have to call, and some arguments to process).
We now support setting up both Celery 3 (on Python 2.7) and Celery 5 (on

Python 3). This code is confined to celery.py, allowing main.py to

be nice and simple. In time, this will also help us upgrade to newer

versions of Celery or to switch to another backend (perhaps

experimentally).

Tested running on Python 2.7 and 3.x in both detached and in-process

modes, with various combinations of options.
Unit tests pass.