New scanpy 2.0 optional dependency extra

[ilan-gold]

I think we definitely need:

1. igraph
2. scikit-misc

And may potentially need:

1. illico

I think a new option like scanpy2 could help smooth this out. We could then raise an error if you, for example, turn on the 2.0 presets without having one of these installed.

scikit-misc is definitely the most worrisome default dependncy given the issues we have had with it in the past. This one may be worth an scverse reimplementation. Or python bindings to https://github.com/thisisamirv/loess-rs (disclaimer, I just googled "loess rust")

[flying-sheep]

Currently it’s a mixed bag between feature-specific extras (scrublet) and extras named after dependencies enabling multiple features (skmisc), that should be standardized. IDK what’s best though as it’s a n-to-n mapping between features and their dependencies which can also change.

PS: I wish PEP 771 (default extras) was accepted already. Would be awesome to have igraph as a default extra so people don’t pip install scanpy>=2 and half of the functions don’t work because they need the igraph extra.