Pyigt

- Dropped support for Python 3.7.
- Added support for Python 3.11 and 3.12.
- Use the LGR CLDF dataset as conformance test suite.
- Added a `pycldf.orm.Example` subclass to easily instantiate `IGT` objects from CLDF data.

- Unified treatment of gloss elements (according to LGR) in `IGT` and `Corpus` objects.
- Docs on rtd

Backward Incompatibilities

`CorpusSpec` has been removed, and consequently `Corpus` and `IGT` do not
accept a `spec` keyword argument anymore.
Removal of `CorpusSpec` may also lead to slightly different output from
the `write_*` methods of `Corpus`.