Remove cached-property dependency

[obi1kenobi]

I think we should remove our dependency on cached-property, for a few reasons:

• We use a very minimal piece of functionality we can easily replicate and improve upon ourselves.
• It isn't type-hinted, and the open issue for it is over a year old with no activity: Support mypy / static typing pydanny/cached-property#172
• The lack of type hints means that we have to always suppress mypy's disallow_untyped_decorators rule. It also means that @cached_property properties return type Any, which makes mypy even less useful.
• @cached_property doesn't inherit from @property, causing a number of other type issues. Here's the tracking issue for it, which has also been inactive in many years: Can cached_property be a subclass of property? pydanny/cached-property#26