Tweak configuration to support partial rebuilds with Sphinx cache

[JasperCraeghs]

Sphinx==7.3.0 reports a new warning when, for example, a function is stored in the configuration dict for the Sphinx application, which breaks the cache functionality of sphinx-build and results in full rebuilds.

sphinx-traceability-extension/mlx/traceability/traceability.py

Line 541 in d4297c0

'effort': natsort.natsorted,

WARNING: cannot cache unpickable configuration value: 'traceability_attributes_sort' (because it contains a function, class, or module object)

Instead of supporting a function, the configuration could work with a string too. 'effort': 'natsort.natsorted', FYI https://stackoverflow.com/a/4605 If someone uses a function, we should report our own warning (on top of Sphinx'), informing the user to use a string instead.

FYI @Letme @JokeWaumans