Investigate Selector.__hash__ implementation

[mzorzo]

Context

This issue came up during the review of the mypy-related changes in PR #140.

Summary

Selector currently defines __hash__ = None, making instances unhashable.

While updating the type annotations, we kept this behavior unchanged because it's not clear whether this was an intentional design decision or whether Selector should implement a proper __hash__ method.

[ocefpaf]

Solved in #159

It turns out the library intends is to leave the Selector class unhashable, so we need to make it None or not define it. we decided to leave things as-is an skip type checking there. See #159 for more info.