Tcod-ec

Fixed
- Corrected the type-hinting of `ComponentDict.get` and `ComponentDict.setdefault`.

Added
- `ComponentDict` is now a `MutableMapping` subclass.
It can now do everything expected of a dict-like object, such as the `.values()` method.
Keep in mind it still uses identity comparison.
- Added local component observers to `ComponentDict`.

Changed
- `abstract_component` is no longer deprecated.

Added
- New `tcod.ec.Composite` class.
Supports multiple components of the same type and looking up components with a parent type.

Fixed
- Better handling of `__dict__` in `ComponentDict` subclasses.
Allowing migration of attributes to or from slots for subclasses the next time they are pickled.

Added
- `ComponentDict.global_observers` for observing changes in components globally.
- `ComponentDict.clear` for removing all components.

Changed
- `ComponentDict` internals have changed. No longer uses attrs and has a less complex constructor.
Should still be able to unpickle plain instances pickled in v1.
- No longer depends on attrs.

Deprecated
- `abstract_component` has been deprecated. Components storing base classes should be used instead.

Added
- `ComponentDict` can now be tested for multiple components at once. Example: `(Foo, Bar) in component_dict`

Added
- ``ComponentDict.set`` now returns `self`.