chore(block-kit): prefer pyproject.toml over requirements.txt

[slack-samples]

Type of change

• Documentation
• Dependency packaging metadata

Summary

Move the Block Kit example from requirements.txt to pyproject.toml so its runtime and development dependencies live in the standard Python project metadata file.

This keeps Dependabot focused on the existing block-kit pip manifest location while letting local and CI setup install the sample with pip install -e ".[dev]". The runtime dependency remains slack_sdk==3.41.0; lint, type-check, and test tools are preserved as the dev extra.

Related discussion: slack-samples/bolt-python-getting-started-app#7 tracks the shared pyproject.toml vs requirements.txt discussion for this Python batch.

Reviewers

Please verify the Block Kit example still installs and runs checks from its nested directory:

$ cd block-kit
$ pip install -e ".[dev]"
$ ruff check
$ ruff format --diff --check
$ mypy ./**/*.py
$ pytest -v

Slack CLI app creation check (using the nested Block Kit example):

$ slack create -t slack-samples/bolt-python-examples -b chore/prefer-pyproject-over-requirements --subdir block-kit

Requirements

• I have ensured the changes align with existing patterns and ran the checks available on this host.
• I've read and agree to the Code of Conduct.

Sources

• PEP 518 defines pyproject.toml as the build-system configuration file for Python projects.
• PEP 621 defines project metadata, including dependencies, in pyproject.toml.
• The PyPA packaging guide documents pyproject.toml as the modern place for project metadata.
• pip supports installing local projects from pyproject.toml, including editable installs with extras.