Support mypy / static typing

[vbraun]

Right now static typing isn't really supported, and we are basically forced to turn it off via

from cached_property import cached_property  # type: ignore

What seems to work pretty well is to pretend to mypy that cached_property is the same as property, that is, write a cached_property.pyi stub file with just

cached_property = property

and then manually point mypy at it. Ideally that would be part of the out-of-the-box experience. For that we would need to either push support into typeshed or:

• Rewrite cached_property.py into a package (code goes into cached_property/__init__.py or import it from there)
• Add the cached_property/__init__.pyi stub
• Add the cached_property/py.typed marker as in https://www.python.org/dev/peps/pep-0561/

With #26 we wouldn't need the stub pyi file but the py.typed marker and refactoring into a package would still be required.