More relaxed `require-subclass` option

[ariebovenberg]

Currently slotscheck has the require-subclass option, which enforces the use of __slots__ wherever slotted classes are subclassed. However, this greatly depends on whether builtin/extension classes are considered "slotted". For example, object has no __dict__. Flagging all direct object subclasses would essentially require slots for all classes in a module (that don't subclass from a non-slot class outside the module).