MAINT: remove dependency on pandas_datareader

This line in `data.py` is causing a FutureWarning

```python
    gdp_df = wb.download(indicator=varlist, country=c, start=s, end=e)
```

which is from `pandas_datareader`. 

This package is now old and I would consider unmaintained so we should migrate this code to use `wbgapi`