Update ubuntu-20.04 to ubuntu-latest in GitHub workflow

[Copilot]

Updates the deprecated ubuntu-20.04 runner to ubuntu-latest in the GitHub Actions workflow to ensure compatibility with GitHub's updated infrastructure.

Changes:

• Updated .github/workflows/tests.yaml line 16: os: ["ubuntu-20.04"] → os: ["ubuntu-latest"]

Benefits:

• Uses the most recent stable Ubuntu LTS version
• Receives the latest security updates and patches
• Includes updated tooling and dependencies
• Future-proofs the workflow against runner deprecation

Before:

matrix:
  os: ["ubuntu-20.04"]
  python_version: ["3.9", "3.10", "3.11", "3.12"]

After:

matrix:
  os: ["ubuntu-latest"]
  python_version: ["3.9", "3.10", "3.11", "3.12"]

This change ensures the Python tests will continue to run successfully on GitHub's supported runner infrastructure.

Fixes #39.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.