Factor out scikit-genome (skgenome) package

[etal]

The functionality in the genome and tabio sub-packages is general-purpose and doesn't need to be tied to cnvlib. Remove their remaining dependencies on the other cnvlib modules, and move to a separate skgenome package and skgenome.tabio sub-package.

Eventually write separate docs and spin off a proper scikit-genome package that can be installed separately from CNVkit.

[etal]

Nah: polars-bio, scikit-bio, and pyranges_1.x cover the space well enough. Better to keep the internal sub-package compact with bioframe (#982) and keep it together with CNVkit.