althonos/pronto: 2.0.0

Added

• TermSet.subclasses and TermSet.superclasses methods to query all
  the subclasses / superclasses of all Term.
• TermSet class to the top-level pronto module.
• Dynamic management of subclassing cache for the Ontology class.
• Setters for Term.consider, Term.union_of and Term.intersection_of. ### Removed
• cache keyword argument for the Ontology. ### Fixed
• SuperclassesIterator.to_set being named to_self because of a typo.
• Several bugs affecting the fastobo-backed serializer.