MNT use pooch instead of unmaintained `download`

[jolars]

https://github.com/choldgraf/download is unmaintained. You might want to consider switching. (The maintainer recommends https://github.com/fatiando/pooch)

[mathurinm]

Good idea @jolars, if you have time to take a look at it 🙏