Add [dev] extras to setup.py; remove stale slodels comment

[Copilot]

README.md documents pip install -e .[dev] for local development, but setup.py had no dev extra defined, making that instruction non-functional. A leftover comment inside extras_require also incorrectly implied slodels required a custom index.

Changes

• setup.py: Remove the outdated # To install slodels, use a custom index comment from the comparative extra
• setup.py: Add a dev extras group with the dependencies used in CI, pinned to match workflow versions:

"dev": [
    "pytest",
    "black==26.1.0",
],

pip install -e .[dev] now resolves correctly and matches what the lint and regression-test workflows install.

• Fixes Dev Mode Support #851