fix(attachments): use proper attribute to retrieve URLs

[delphiki]

I have done all the following:

• added the feature
• added documentation for the feature
• added tests for the feature
• ran mypy, black, and unit tests

Attachments of type "URL" follow this schema (at least on the Pronote version used by the school of my kids):

"ListePieceJointe": {
  "_T": 24,
  "V": [
    {
      "N": "XXXXXXXXXX",
      "G": 0,
      "url": "https://digipad.app/p/393............"
    },
    {
      "L": "Cours Nuit et Brouillard 2023.pdf",
      "N": "XXXXXXXXXX",
      "G": 1
    },
    {
      "L": "Fiche Nuit et Brouillard.pdf",
      "N": "XXXXXXXXXX",
      "G": 1
    }
  ]
}

[delphiki]

Merci pour tes corrections :)