Add PyPI distribution support

[BHFock]

Publishes git-cl to PyPI as git-changelists, making it installable via pip:

pip install git-changelists

Changes

• Add setup.cfg with full package metadata
• Add pyproject.toml with build system configuration
• Update README.md with pip install instructions and PyPI badge
• Bump version to 1.1.3 across git-cl, setup.cfg, and CITATION.cff

Notes

• The wget install method is unchanged and continues to work as before
• Package name is git-changelists due to an abandoned occupant on PyPI holding the git-cl name
• No changes to the script itself