Fastobo

Added
- `__init__` implementation for `fastobo.term.DefClause`
- `__init__` implementation for `fastobo.syn.Synonym`
- Precompiled wheels for PyPy3 on OSX and Linux.
Fixed
- `XrefList.__repr__` implementation entering infinite recursion.
- Derive macros generating weird error messages for some `TypeError`s.
- Enabled `extension-module` feature of `pyo3` to allow static linking to Python interpreter.
- Inconsistent error-chaining in Python causing issues with `try/except` blocks.

Changed
- Use stable PyO3 release (`v0.8.0`).
- Add back `__build__` attribute to check build variables.

Added
- Added BOSC 2019 poster reference to `README.md`.
- Added `__richcmp__` implementation to `fastobo.id.PrefixedIdent`.
Changed
- Bumped Rust dependencies to latest PyO3 version.

Fixed
- `PyFile.write` calling `write` with two arguments causing duck typing check
to fail all the time in `dump_graph`.

Added
- `load_graph` and `dump_graph` functions to read and write OBO JSON files.
Changed
- Updated to `fastobo` [v0.7.1](https://github.com/fastobo/fastobo/blob/master/CHANGELOG.md#v071---2019-08-08).

Changed
- Updated to `fastobo` [v0.6.1](https://github.com/fastobo/fastobo/blob/master/CHANGELOG.md#v061---2019-07-24).
- Updated to `url` v2.0.0.