Prepare release 1.3.1

[carluri]

Summary

Prepares RT-Utils 1.3.1 for publication to PyPI after the voxel-edge contour
feature was merged.

Version and metadata consistency

• Adds rt_utils/_version.py as the package version source of truth.
• Exposes rt_utils.__version__.
• Updates package metadata to 1.3.1.
• Corrects the repository homepage URL.
• Aligns declared Python support with the tested CI floor by requiring Python
  3.8+.

Packaging fixes

Release validation exposed two existing source-distribution defects:

• The previous package_dir/find_packages combination omitted package source
  files from the sdist.
• requirements.txt, which setup.py reads, was omitted from the sdist.

This PR fixes package discovery and adds MANIFEST.in so both the sdist and
wheel can be built from the generated sdist.

Release process

Adds RELEASING.md with an explicit test, build, upload, tag, and GitHub-release
sequence to prevent GitHub/PyPI version drift.

Validation

• Full test suite: 46 passed.
• python -m build: passed for sdist and wheel.
• python -m twine check: passed for both artifacts.
• Verified artifact metadata: name rt-utils, version 1.3.1, Python >=3.8.
• Verified package source and build inputs exist in the sdist.
• Installed the built wheel and verified both rt_utils.__version__ and
  importlib.metadata.version("rt-utils") equal 1.3.1.

[qurit-frizi]

approved

[qurit-frizi]

approved

approved