Fix update-contributors workflow by using a workflow-specific requirements file

[Copilot]

The Update Contributors List workflow fails during dependency installation: tools/requirements.txt pins PyYAML==6.0, which cannot build from source on Python 3.12.

Collecting PyYAML==6.0 (from -r tools/requirements.txt (line 1))
  ...
  AttributeError: 'build_ext' object has no attribute 'cython_sources'
ERROR: Failed to build 'PyYAML' when getting requirements to build wheel

tools/requirements.txt is consumed by offline processes, so its pins are left alone.

Changes

• .github/workflows/requirements.txt (new): workflow-only dependency set, containing just requests>=2.32.4. tools/update_contributors.py imports only requests; PyYAML was never needed here.
• .github/workflows/update-contributors.yml: install from the new file instead of tools/requirements.txt.

- name: Install dependencies
  run: pip install -r .github/workflows/requirements.txt

Follows the existing precedent of docs/requirements.txt for context-specific dependency sets.