Support for PEP 751

[woodruffw]

We should evaluate support for PEP 751!

More specifically: PEP 751 explicitly includes support for attestation identities via [[packages.attestation-identities]].

For example:

lock-version = '1.0'
environments = ["sys_platform == 'win32'", "sys_platform == 'linux'"]
requires-python = '==3.12'
created-by = 'mousebender'

[[packages]]
name = 'attrs'
version = '25.1.0'
requires-python = '>=3.8'
wheels = [
  {name = 'attrs-25.1.0-py3-none-any.whl', upload-time = 2025-01-25T11:30:10.164985+00:00, url = 'https://files.pythonhosted.org/packages/fc/30/d4986a882011f9df997a55e6becd864812ccfcd821d64aac8570ee39f719/attrs-25.1.0-py3-none-any.whl', size = 63152, hashes = {sha256 = 'c75a69e28a550a7e93789579c22aa26b0f5b83b75dc4e08fe092980051e1090a'}},
]
[[packages.attestation-identities]]
environment = 'release-pypi'
kind = 'GitHub'
repository = 'python-attrs/attrs'
workflow = 'pypi-package.yml'

We should be able to consume a PEP 751 lockfile, convert its attestation identities into appropriate policies, and verify against them.