Update to Python 3.15

[hingebase]

Related changes in Python 3.15:

• PEP 810
  In src/mahoraga/_cli/__init__.py: lazy from mahoraga import _core
• gh-140448: Default suggest_on_error to True in argparse.ArgumentParser python/cpython#140450

Related metadata:

• .python-version
• pyproject.toml
  • build-system.requires
  • project.requires-python
  • project.classifiers