Basicco

Fixed:
- Install requires

Changed:
- Use `tippo`

New:
- Major version up (backwards incompatible)
- Drop compatibility with Python 2 and older versions of Python 3
- Minimum requirement is Python 3.7+
- Simplify, remove unnecessary code

Removed:
- Frozen functionality

Added:
- `is_iterable` and `assert_is_iterable` functions

Added:
- `utils.sentinel_value`
- `utils.value_factoring`