Code cleanup for 2.0 release

[reece]

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