Pronto

Added
- Setters for the `holds_over_chain` and `equivalent_to_chain` properties of `Relationship` objects.
Fixed
- Serialization of `holds_over_chain` properties with `fastobo`-based serializers ([175](https://github.com/althonos/pronto/issues/175)).

Fixed
- Serialization of *metadata tag* relationships by `fastobo` ([164](https://github.com/althonos/pronto/issues/164)).

Added
- `OwlFunctionalSerializer` to dump an `Ontology` to OWL Functional-style syntax.
Changed
- Bumped `fastobo` dependency to `v0.11.1`.
- Make `FastoboParser` raise a `SyntaxWarning` when encoutering creation dates that are not `datetime.datetime`.

Added
- Missing documentation for the `pronto.Entity.relationships` property.
Fixed
- RDX/XML parser crashing on files containing a relationship and a term with the same ID ([138](https://github.com/althonos/pronto/pull/138)).

Added
- Support for `chardet` version `4.0` (in addition to older `3.0`).
Fixed
- Serialization of `Ontology` failing with non-empty `idspaces`.
- Typo in OWL2 URL in `README.md` ([130](https://github.com/althonos/pronto/issues/130)).

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`.