[WIP] update snakemake version

[ctb]

This PR updates snakemake to 6.10.0.

There are a few other misc changes -

• limit Python to < 3.10 for now
• add a print statement that makes genome download clearer

Maybe TODO:

• [ ] update sourmash to 4.2.2 (?)
• [ ] switch to using sourmash sketch rather than sourmash compute