2.4.1

Changed

• pronto.pv.PropertyValue is now an abstract class.
• pronto.parsers.RdfXmlParser now ignores synonym Xrefs not in
  the right format.

Fixed

• pronto.Entity.definition documentation now lists return type as
  pronto.definition.Definition as expected.
• CURIE compaction in RDF/XML not being handled consistently, causing
  some crashes on ontologies using aliased relationships.
• pronto.utils.typechecked.disabled is now reentrant and should
  not be disabled in multithreaded contexts anymore.

Removed

• Implicit injection of lxml instead of xml.etree, which caused
  issues with RdfXmlParser.