import warning

Doesn't impact usage, but appears every time shelley-bio is called:

```
/usr/lib/python3.10/runpy.py:126: RuntimeWarning: 'shelley_bio.client.cli' found in sys.modules after import of package 'shelley_bio.client', but prior to execution of 'shelley_bio.client.cli'; this may result in unpredictable behaviour
  warn(RuntimeWarning(msg))
```