Upgrade to Leaflet v1.6.0

[Conengmo]

Leaflet v1.6.0 has been released.

https://github.com/Leaflet/Leaflet/releases/tag/v1.6.0

• Check the release notes for changes that may impact us.
• Bump the version and test if everything still works.
• Merge the PR

[ocefpaf]

According to https://github.com/Leaflet/Leaflet/releases/tag/v1.6.0 there are two API changes that I don't think will affect folium.