Pronto

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.

Changed
- Bumped `fastobo` to `v0.7.0`, switching parser implementation to use
multi-threading in order to speedup the parser process.

Added
- Explicit support for Python 3.8.
- Support for Windows-style line endings
([53](https://github.com/althonos/pronto/issues/53))

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.