Improve RuleGroup typing

[trag1c]

RuleGroups currently cannot be parametrized and so all rules it contains are marked as Rule[Any]:

class Rule:
    ...
    def __or__(self, other: object) -> RuleGroup:
        if isinstance(other, Rule):
            return RuleGroup((self, other))
        if isinstance(other, RuleGroup):
            return RuleGroup((self, *other.rules))
        return NotImplemented


@dataclass(frozen=True)
class RuleGroup:
    rules: tuple[Rule[Any], ...]

    def __iter__(self) -> Iterator[Rule[Any]]:
        ...

    def __or__(self, other: object) -> RuleGroup:
        if isinstance(other, RuleGroup):
            return RuleGroup((*self.rules, *other.rules))
        if isinstance(other, Rule):
            return RuleGroup((*self.rules, other))
        return NotImplemented

So far I've struggled to find a way of making this type generic properly.

This issue is related to #15 as this makes it harder to correctly type the Crossandra.__rules attribute.