Slotted

Added:
- Add compatibility with Python 2.7 and support for generic collections

Changed:
- Remove makefile
- Add py.typed

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

Fixed:

- __init_subclass__ method behavior

Added:

- Add `tox` to `dev_requirements`
- More badges

Fixed:

- Missing files and docs from git