Pronto

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`.

Added
- Deprecation warnings for the retrieval of relationships via
indexing, to be deprecated in `v3`.
Changed
- Replaced Travis-CI with GitHub Actions to handle continuous integration.
- Bumped `fastobo` dependency to `v0.10.0`.
Removed
- Retrieval of terms via their alternate IDs (introduced in `v2.3.0`,
caused multiple issues ([120](https://github.com/althonos/pronto/issues/120),
[126](https://github.com/althonos/pronto/issues/126)).

Added
- Support for path-like objects when creating an `Ontology`
([108](https://github.com/althonos/pronto/pull/108)).
Fixed
- Avoid decoding file-like objects if they are already **UTF-8**
encoded when creating an `Ontology`
([110](https://github.com/althonos/pronto/pull/110)).

Fixed
- `pronto.entity` package not being included in source distribution.