Teletype

- Drops Python <3.7 support
- Adds type annotations
- Adds mypy support
- Moves to a declarative setup configuration

Component Refactoring
- gets rid of relative imports
- follows a more object-oriented design; all components extend `Component` base class
- `SelectOne` is now just `Select`
- otherwise mostly the same interface, see readme for details

**Note:** Until version 1.0 there is a possibility of further breaking component changes

Bug Fixes
- 1 Fixes off by one selection error for SelectOne and SelectMany components
- 2 Documents `teletype.components.config.ascii_mode` in readme

pre-release 🎉