1.2.0

Added

• Parameter with_self to disable reflexivity of Term.subclasses and
  Term.superclasses iterators.
• TermSet class which stores a set of terms efficiently while providing
  some useful shortcuts to access the underlying data.

Changed

• Moved code of Term.subclasses and Term.superclasses to a dedicated
  iterator class in the pronto.logic submodule.
• Dropped contexter requirement.

Fixed

• Fix a typo in Synonym.type setter leading to a potential bug when
  the given type is None.
• Fix miscellaneous bugs found with mypy.
• fastobo serializer crashing on namespace clauses because of a type
  issue.
• fastobo parsers using data version clauses as format version clauses.