add pip-audit

[rsyring]

@nox.session
def audit(session: nox.Session):
    # Much faster to install the deps first and have pip-audit run agains the venv
    session.install('-r', 'requirements/dev.txt')
    session.run(
        'pip-audit',
        '--desc',
        '--skip-editable',
    )

And add pip-audit to dev.in