fix pandas warning

[sapountzis]

Currently the groupby raises a warning by pandas as the default behavior will change

From pandas documentation:

observed : bool, default False
This only applies if any of the groupers are Categoricals. If True: only show observed values for categorical groupers. If False: show all values for categorical groupers.

Deprecated since version 2.1.0: The default value will change to True in a future version of pandas.

To retain the old behavior we need to set observed=False which also fixes the warning

[mbaak]

Thank you! Will have a quick test run and merge :)

[sapountzis]

Thanks! Any plans for a new release in pip?

[mbaak]

We're working on it :)