chore: prefer pyproject.toml over requirements.txt

[slack-samples]

Type of change

• Documentation

Summary

This PR continues our migration away from requirements.txt and toward pyproject.toml as the single source of dependency and packaging metadata.

Using pyproject.toml improves consistency between local setup and CI, reduces duplicated dependency definitions, and keeps optional/dev dependencies structured in one place.

Reviewers

Use this command to create a fresh local copy on the PR branch:

$ slack create -t slack-samples/bolt-python-starter-agent -b chore/prefer-pyproject-over-requirements

Manual checks to perform:

• Confirm there is no remaining requirements.txt dependency path in docs or CI for the changed scope.
• Confirm install commands now use pyproject.toml-based flow (e.g., pip install . or pip install -e ".[dev]").
• Confirm CI workflow changes align with the repo’s Python version matrix and still pass.

Discussion

Please use bolt-python-getting-started-app #7 for cross-repo discussion.

Repo-specific changes

• Removed per-subdir requirements.txt files from:
  • claude-agent-sdk/requirements.txt
  • openai-agents-sdk/requirements.txt
  • pydantic-ai/requirements.txt
• Updated each subdir README install command from pip install -r requirements.txt to pip install -e ".[dev]"
• Updated matrix CI workflows (.github/workflows/tests.yml, .github/workflows/ruff.yml) to install from pyproject.toml metadata in each subdir

Sources

• PEP 518 – Specifying Minimum Build System Requirements for Python Projects
• PEP 621 – Storing project metadata in pyproject.toml
• PyPA Packaging Guide – Writing your pyproject.toml
• pip docs – Local project installs (including editable installs)

Requirements

• I have ensured the changes I am contributing align with existing patterns and have tested and linted my code
• I've read and agree to the Code of Conduct