Code cleanup for 2.0 release

This issue collects changes to consider for the next major release.

See also the 2.0 milestone

Issues will be made for any changes that we commit to.

Ideas:

• [ ] ~Investigate whether intervalmapper, transcriptmapper, and variantmapper are obsolete~.
• [ ] move models to hgvs.models.*. (#583)
• [ ] make seqrepo dependency optional
• [x] cleanup hgvs.easy
• [x] remove six
• [x] ~remove future imports~ (#688)
• [x] code reformat
• [x] code restructure (src/, setup.cfg)
• [ ] verify that all dependencies are up-to-date. e.g., see #584
• [ ] improve consistency of error handling, esp. returning variant as-is v. exceptions when error is encountered (code currently does both)

External needs:

• [ ] SeqRepo raises error when coordinates are out-of-bounds
• [ ] SeqRepo make metadata accessible

• [ ] Remove libpq
• [ ] Default to rest
• [ ] Add uncertainty change (as a carrot to encourage upgrade)
• [ ] Drop Python 3.10 support and slide to versions to 3.11, 3.12, 3.13

DataProvider API changes

We can add placeholders here, even if we don't populate them yet

• [ ] CDS Phase - see #732
• [ ] Canonical / Tags - see #747
• [ ] Remove mode etc lru_cache #784