Pronto

Fixed
- Handling of some clauses in `FastoboParser`.
- `OboSerializer` occasionaly missing lines between term and typedef frames.
Added
- Missing docstrings to some `Entity` properties.

Fixed
- `RdfXMLParser` crashing on entities with `rdf:label` elements
without literal content.

Fixed
- `pronto.serializers` module not being embedded in Wheel distribution.

Added
- `Entity.add_synonym` method to create a new synonym and add it to an entity.
- `roundrepr` now adds a minimal docstring to the generated `__repr__` method.
- `Ontology` caches subclassing relationships to greatly improve performance of
`Term.subclasses`.
Changed
- `Entity` subclasses now store their `id` directly to improve performance.
- `Term.subclasses` and `Term.superclasses` use `collections.deque` instead of
`queue.Queue` as a LIFO structure since thread-safety is not needed.
- `chardet` result is now used even when prediction confidence is under 100%
to detect encoding of the handle passed to `Ontology`.
Fixed
- `SynonymType` comparison implementation.
- `Synonym.type` getter crashing on `type` not being `None`.
- `RdfXMLParser` crashing on synonymtypedefs without scope specifiers.

Fixed
- Issues with typedef serialization in `FastoboSerializer`.
- `Ontology.create_term` and `Ontology.create_relationship` not raising `ValueError`
when given an identifier already in the knowledge graph.
- Signature of `BaseSerializer.dump` to remove `encoding` argument.
- Missing `__slots__` in `Entity` in non-typechecking runtime.
Changed
- Bumped `fastobo` requirement to `v0.6.0`.

Added
- Extraction of `oboInOwl:consider` annotation in `RdfXMLParser`.
- Extraction of `oboInOwl:savedBy` annotation in `RdfXMLParser`.
- Extraction of `subsetdef` and `synonymtypedef` as annotation properties in
`RdfXMLParser`.
- Support for `doap:Version` instead of `owl:VersionIri` for specification
of ontology data version.
- Proper comparison of `PropertyValue` classes, based on the lexicographic order
of their serialization.
- `Ontology.dump` and `Ontology.dumps` methods to serialize an ontology in
**obo** or **obojson** format.
Fixed
- `Metadata` not storing optional description of ID spaces if any.
- Wrong type hints in `RelationshipData.equivalent_to_chain`.
Changed
- Added type checking to some more property setters.
- Avoid using `networkx` in `Term.subclasses`.
- `fastobo`-derived parsers will not create a new entity if one exists in the
graph of dependencies already.
- Exposed `pronto.warnings` and the complete warnings hierarchy.