py.typed marker is not present and reader_class annotation is incorrect

[robinmackaij]

The py.typed marker has not been added to the repo, making mypy consider the repo untyped (while it is typed).

With the py.typed in place, subclassing the ReaderClass and passing such an implementation to the reader_class init parameter will yield a type error; the reader_class is typed to expect an instance of the AbstractReaderClass instead of an instance of a subclass (Type[AbstractReaderClass]).