Fix linted issues

[LucaMantani]

This PR fixes some issues in the code that are now linted with the new version of pylint: 3.2.0. It also requires a more updated version of seaborn, so that it is compatible with the latest version of matplotlib 3.9.0.

Issues were:

************* Module smefit.analyze.fisher
src/smefit/analyze/fisher.py:165:41: E0606: Possibly using variable 'axis_sum' before assignment (possibly-used-before-assignment)
src/smefit/analyze/fisher.py:165:57: E0606: Possibly using variable 'axis_div' before assignment (possibly-used-before-assignment)
************* Module smefit.optimize.ultranest
src/smefit/optimize/ultranest.py:179:12: E0606: Possibly using variable 'loaded_datasets' before assignment (possibly-used-before-assignment)
************* Module smefit.postfit
src/smefit/postfit/__init__.py:92:34: E0606: Possibly using variable 'coeffs' before assignment (possibly-used-before-assignment)

[jacoterh]

Looks good, thanks for adding these fixes. I'd say we merge as soon as we got python 3.12 to work as well, as per issue #78

[LucaMantani]

Something is weird in the tests btw, there are outputs like this one:

• Requirement already satisfied: packaging in /usr/share/miniconda/envs/smefit_installation/lib/python3.10/site-packages

for all of them, even if the version of python that is being tested is not 3.10...