Error: Use either --since or --since_id, not both - still broken

Hi Simon,

It appears the fix for #57 doesn't fix things for me:

```
$ twitter-to-sqlite --version
twitter-to-sqlite, version 0.21.4
$ python --version
Python 3.9.6
```

```
$ twitter-to-sqlite home-timeline -a twitter-auth.json twitter/timeline.db --since
Importing tweets
Error: Use either --since or --since_id, not both
```

Is there any way I can help debug this?