TaxBenefitSystem.load_variable() and TaxBenefitSystem.update_variable() are improperly typehinted

[anth-volk]

At least, I think. For example, if instantiating a new tax-benefit system (TBS), then using .update_variable() to load off based on a string (as hinted in -core), the underlying .load_variable() method will throw an error indicating that a str type has no property __name__, even though both methods are hinted to take strings. I believe they're actually meant to take Variable-types.