Add Python 3.15 support

[CarlKCarlK]

Track support for Python 3.15 in PySnpTools.

Scope:

• Update package classifiers and constraints.
• Add Python 3.15 to CI and artifact-install coverage.
• Run representative BED/SNP and association workflows, including compatibility with the supported bed-reader release.

Version policy:

• Target Python 3.12-3.15 for the release adding Python 3.15.
• Drop Python 3.10, whose end of life is expected around October 2026, and Python 3.11 in line with Scientific Python SPEC 0's three-year support window.
• Revisit the floor annually: add the newest Python and drop versions outside that three-year window.
• Reassess dependency minimums and remove compatibility constraints that exist only for Python 3.10 or 3.11.