Add slotscheck

[ItsDrike]

Currently, the only thing enforcing __slots__ in the code-base is the RequiredParamsABCMixin class, which is only checking for presence of the variable and prevents initialization if this presence check fails (similarly to how ABCs work).

However there's a lot more things that we should be enforcing when it comes to __slots__, such as avoiding duplicates, ensuring all subclasses have __slots__ defined, etc. For these additional checks, we can use the slotscheck library.

Along with that, slotscheck can also require all classes to be slotted by default, and produce a linter error otherwise. Of course, we can always add specific ignores if some class shouldn't be slotted (though slotscheck is actually pretty good at recognizing those cases itself, so it might not even be that necessary).

However since slotscheck works by actually running the code it's checking, and performing it's checks on runtime, some of it's edge case ignores that it provides require proper typing imports (like to ignore the need for __slots__ in TypedDict or Protocol classes), which is why this PR also converts our current way of using typing-extensions and turns it into a full runtime dependency.

This is not a huge issue, as it seems that one of our dependencies already requires typing-extensions on runtime, so us requiring it too doesn't actually change anything about whether the library users will end up installing it.