Abbreviate type aliases in API

[redeboer]

Signatures that use type aliases look rather ugly in the existing APIs of QRules, AmpForm, and TensorWaves.

This can be fixed with autodoc_type_aliases. The problem though, is that this requires from __future__ import annotations, which is only available in Python >3.7. Alternatively, type aliases can be abbreviated by making the signatures strings, as was done (by accident due to OrderedDict) here:

See also #58

[redeboer]

Done for AmpForm and TensorWaves (ComPWA/ampform#207 and ComPWA/tensorwaves#404). QRules still proved problematic, but this may improve once Sphinx v4.5.0 is out (ComPWA/qrules#156 (comment)).

[redeboer]

Works well, see e.g. https://ampform.readthedocs.io/en/0.14.x/api/ampform.dynamics.html#ampform.dynamics.EnergyDependentWidth. For QRules, this will be automatically addressed through ComPWA/.github#5 once it drops support for Python 3.6.