Adding `security` as a well-known URL label and `[project.security]` table to pyproject.toml

Hi all, I’ve been contributing to CPython’s security tooling and Python docs translation, and I keep running into the same practical gap: there is no standard, machine-readable way for a Python package to declare its security contact, policy, or disclosure method. Checking the current well-known URL labels (from PEP 753): homepage, source, download, changelog, releasenotes, documentation, issues, funding — security is not on this list. Package authors who add "Security Policy" = "..." to their...