Simplify abbreviate_signature.py script

[redeboer]

QRules, AmpForm, and TensorWaves build their API with a small hack: abbreviate_signature.py, which is called through conf.py. This script does two things:

1. It remaps broken intersphinx links.
2. It abbreviates types in signatures.

The first can be simplified with dict.get. The second can be removed altogether with autodoc_typehints_format = "short", which is available since Sphinx v4.4.0.

Warning — Sphinx v4.4.0:

• places constraints on importlib-metadata, which causes dependency conflicts for Python <=3.7.
• changes the signature of sphinx.domains.python.type_to_xref, which is substituted through abbreviate_signature.py here.

See also #59

[redeboer]

Closed by the new sphinx-api-relink extension. See for instance ComPWA/ampform#370.