2.0.1

Fixed

• Internal handling of ontology data forcing an Ontology to outlive all
  of the Terms created from it.
• Term.id property missing a return type annotation.
• Term.equivalent_to not returning a TermSet but a set of strings.

Changed

• Refactored implementation of SubclassesIterator and
  SuperclassesIterator to make both use the interal subclassing cache.
• Make Term.is_leaf use internal subclassing cache to make it run in
  constant time.