Test failure: ValueError: Custom tiles must have an attribution.

[davidwagner]

When running tests locally, they fail with the following error:

___________________________ tests/Maps.ipynb::Cell 3 ___________________________
Notebook cell execution failed
Cell 3: Cell execution caused an exception

Input:
# Different tile styles
Map(tiles='Stamen Toner').show()
Map(tiles='Stamen Terrain')
[...]
ValueError: Custom tiles must have an attribution.

I suspect this is because I'm running folium 0.20.0 locally, but the cloud continuous integration environment is running an earlier version of folium (0.11.0?). Notice requirements.txt (what I use locally) has folium>=0.9.1, whereas linux_environment.yml has folium==0.11.0 (what is used in the cloud continuous integration environment, I think).

Perhaps 'Stamen Terrain' might need to change to 'Stadia.StamenTerrain', too.

I'm not sure whether we should continue to use Stamen tiles, at least in tests: https://stackoverflow.com/q/77438168/, https://stackoverflow.com/q/52170918/. Available tile providers: https://leaflet-extras.github.io/leaflet-providers/preview/.