[#15] Added generic type hints 🧞

[todofixthis]

closes #15 (thanks @VRichardJP for suggesting!)

⚠️ Backwards-incompatible changes

• Removed BaseRegistry.__dir__(). Use BaseRegistry.keys() instead.
• BaseRegistry.__iter__ now returns an iterator instead of a generator.
• Removed BaseMutableRegistry.__setitem__(). Use BaseMutableRegistry.register() or RegistryPatcher instead.
• Removed BaseMutableRegistry.__delitem__(). Use BaseMutableRegistry.unregister() instead.

MRO changes

• BaseRegistry no longer extends typing.Mapping (it now extends typing.Container).
  • The signatures of BaseRegistry.items() and BaseRegistry.values() conflict with those of the corresponding methods in typing.Mapping, thereby violating the Liskov Substitution Principle.
• BaseMutableRegistry no longer extends typing.MutableMapping.
  • The signature of BaseMutableRegistry.__setitem__() conflicts with that of the corresponding method in typing.MutableMapping, thereby violating the Liskov Substitution Principle.

Changed signatures

These changes only impact your code if you've created classes that derive from ClassRegistry and override the following methods. If your code is impacted, it shouldn't generate any runtime errors, but it may cause your IDE to warn about incompatible signatures and/or generate mypy errors.

• ClassRegistry.__missing__ is now expected to return a class (or raise an exception); previously it could return any type, including None.
• ClassRegistry.get_class is now expected to return a class; previously it could return any type, including None.
• SortedClassRegistry now defines keys() instead of items().

If your code is impacted by the above changes, please post an issue describing your use case, so that I can have a look.

⚠️ Deprecations

• BaseRegistry.values() is deprecated and will be removed in a future version of ClassRegistry. Use BaseRegistry.classes() instead.
• BaseRegistry.items() is deprecated and will be removed in a future version of ClassRegistry. Use zip(registry.keys(), registry.classes()) instead.

Major changes

• ClassRegistry now accepts an optional type parameter, to indicate the type of objects that it can create. Example:

  registry = ClassRegistry[Pokemon]()
  ...
  # IDE will infer that `fighter` is a `Pokemon` instance.
  fighter = registry['grass']

• ClassRegistryInstanceCache now accepts an optional type parameter, or infers it from the ClassRegistry that it wraps. Example:

  registry = ClassRegistry[Pokemon]()
  pokedex = ClassRegistryInstanceCache(registry)
  ...
  # IDE will infer that `fighter` is a `Pokemon` instance.
  fighter = pokedex['grass']
  
  # This also works:
  pokedex = ClassRegistryInstanceCache[Pokemon](registry)

Minor changes

• ClassRegistryInstanceCache now extends typing.Mapping.
• Added ClassRegistryInstanceCache.__len__().
• BaseRegistry.__len__() is no longer abstract.
• Cleared out some obsolete backwards-compatibility logic.
• Integrated mypy into autohooks and CI, and fixed mypy-reported issues.