migrate packaging to `pyproject.toml`

[rsgalloway]

Description

envstack currently uses a legacy setup.py-based packaging configuration.
To improve standards compliance, tooling compatibility, and discoverability, we should migrate packaging metadata to pyproject.toml (PEP 517 / PEP 621).

This migration should be packaging-only and must not change runtime behavior.

Current setup.py for reference:

• name: envstack
• version: 0.9.x
• package root: lib/
• console scripts:
  • envstack=envstack.cli:main
  • whichenv=envstack.cli:whichenv
• dependencies:
  • PyYAML>=5.1.2
  • cryptography>=43.0.1
• Python requirement: >=3.6

Scope

1. Add pyproject.toml

• Define build system (setuptools + wheel)
• Migrate all metadata currently defined in setup.py:
  • name
  • version
  • description
  • license
  • authors
  • classifiers
  • Python version requirement
  • dependencies
  • console scripts
• Use README.md as the long description

2. Preserve existing package layout

• Continue using lib/ as the package root
• Ensure find_packages() equivalent behavior is preserved

3. Verify packaging behavior

• pip install .
• pip install -e .
• python -m build (sdist + wheel)
• Console entrypoints work:
  • envstack --help
  • whichenv

4. Handle setup.py

• Either:
  • Remove setup.py entirely, or
  • Keep a minimal compatibility shim if required
• Decision should favor simplicity and standards compliance

Acceptance Criteria

• pyproject.toml fully replaces setup.py for packaging metadata
• No change in runtime behavior or CLI interface
• Editable installs work as before
• Wheel and sdist build successfully
• PyPI metadata renders correctly (README, classifiers, URLs)

Notes

• This change is intended to improve modern tooling support and discoverability
• Keep the migration strictly scoped to packaging
• Do not refactor code, imports, or CLI logic as part of this issue

[rsgalloway]

Stretch goals:

• Add project.urls (Docs, Source, Issues)
• Add keywords to improve PyPI search
• Add CITATION.cff
• Confirm PyPI description matches README exactly