Modernize SelectorList-related code

[wRAR]

There were multiple issues identified with the code of SelectorList itself, Selector.selectorlist_cls and their typing. Ideally:

• SelectorList should only be able to contain Selector objects
• SelectorList subclasses made to work with Selector subclasses should only able to contain those
• Selector subclasses shouldn't need to set selectorlist_cls to a respective SelectorList subclass manually
• all of this should be properly typed without need for casts and other overrides

This may require changing Selector and/or SelectorList base classes, but I think we will need to keep the API compatibility? It's also non-trivial because the API for subclassing them doesn't seem to be documented, the only reference is SelectorTestCase.test_extending_selector() (the related code was also changed when adding typing, not sure if it changed the interface).