Pynamodb-mypy

What's Changed
* Support derived model initializers by ikonst in https://github.com/pynamodb/pynamodb-mypy/pull/4

**Full Changelog**: https://github.com/pynamodb/pynamodb-mypy/compare/v0.1.1...v0.1.2

What's Changed
* Handling overridden `Model.__init__`

What's Changed
- `Model.__init__` calls are now type-checked: attributes types will have to match and undefined attributes will not be allowed.
- Lowest supported Python version is now 3.8

Internals
- Reworked attribute initialization parsing. Results are now persisted in the model type's mypy metadata.