Support python 3.8

[mattijn]

It would be great if we can align altair_tiles with the same minimal Python dependency as Altair itself (Python 3.8).

See also this commit vega/altair@cd78a24 in this PR vega/altair#3354 where altair_tiles is removed again.

[binste]

I probably have some time this weekend to look into it but I think it should be possible! It's probably just the type hints which need to be adjusted, maybe ruff will be able to do this automatically for me...