fix: using file url as mapping

[nichmor]

This fixes the issue reported by our user : https://discord.com/channels/1082332781146800168/1276948298330275922/1276948298330275922

when using this mapping setup :

[tool.pixi.project.conda-pypi-map]
conda-forge = "file:///path/to/parselmouth/conda-forge.json"

pixi will error out because we don't understand file url scheme.

  × failed to download pypi mapping from file:///path/to/parselmouth/conda-forge.json location
  ├─▶ builder error for url (file:///path/to/parselmouth/conda-forge.json)
  ╰─▶ URL scheme is not allowed

This PR aims to fix it.

[baszalmstra]

From the name of the function I would expect this to return a mapping but instead it returns anything that is deserializable. Imo we can just remove the generics.

[ruben-arts]

@nichmor Can you fix the test