Add python 3.13 support (+ minor docstring improvements)

[eneelo]

What's included in this PR

• Add python 3.13 support, to keep up with the official Status of Python versions.
• Add python 3.13 to test matrix.
• Update python version support info in README.md and documentation.
  The documentation now states how to find information about supported python versions, without stating specifically which versions these are. This is to avoid the need to update this information several places when python version support changes.
• Minor docstring improvements to qats.signal, qats.fatigue.rainflow and qats.stats.

Edit, 08.01.2025:

• Latest release of pyside6 (version 6.8.1.1) breaks installation, because poetry cannot find an installation candidate for shiboken6 version 6.8.1.1 (and pyside6 and shiboken6 version must be identical, ref. https://doc.qt.io/qtforpython-6/shiboken6/gettingstarted.html. A temporary fix is to require pyside6 <= 6.8.1 in pyproject.toml, thereby avoiding version 6.8.1.1. A looser dependency, allowing for >=6.8.1, may be specified when this issue has been fixed on the pyside/shiboken side. This fixes Broken dependency for python 3.12 #134.
  Note: shiboken6 6.6.x (and therefore pyside6 6.6.x) doesn't seem to work well with numpy 2.x, so it is desirable to allow for shiboken6 6.8.x also for python <=3.12 instead of restricting the numpy version more than needed.

To be discussed:

• Drop support for python 3.8? On one side, it is no longer officially supported according to https://devguide.python.org/versions/. On the other hand, it does not (yet) cause any issues to include support for 3.8.
  Edit (08.01.2025): poetry version 2.0 (latest) requires python 3.9+, causing failed tests for python 3.8 (see https://github.com/dnvgl/qats/actions/runs/12666602368/job/35298567574?pr=133). Options: 1) drop support for python 3.8, or 2) ensure in test.yaml that poetry 1.8 is installed for the python 3.8 tests (not sure if possible), or 3) specify poetry 1.8.5 in test.yaml (ref. https://github.com/snok/install-poetry).
  Currently in favor of option 1 (drop python 3.8 support).

Notes

• pyproject.toml: the dependencies under tool.poetry.dependencies have been updated to work across python versions 3.8 through 3.13. It is probably possible (and certainly desirable) to clean up and simplify the dependency spec, which currently includes quite a few statements like "this version for python <= 3.12, that version for python 3.13", etc. At this stage, however, the main goal has been to ensure that poetry install works across the supported python versions (and of course, that pytest and qats app may thereafter be successfully executed).

[eneelo]

@tovop No haste in getting this PR through. We will do it when time allows.

[tovop]

An intense work week but I will attempt a review on Friday.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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