OICA returns 404 for certain data files

[khaeru]

In #51, it was discovered that OICA no longer serves some files, leading to 404 errors:

FAILED transport_data/tests/test_oica.py::test_convert[SALES-N_obs1] - requests.exceptions.HTTPError: 404 Client Error: Not Found for url: https://oica.net/wp-content/uploads/cv_sales_2018.xlsx
FAILED transport_data/tests/test_oica.py::test_convert[STOCK-N_obs2] - requests.exceptions.HTTPError: 404 Client Error: Not Found for url: https://oica.net/wp-content/uploads/CV-World-vehicles-in-use-2020.xlsx
FAILED transport_data/tests/test_oica.py::test_convert[STOCK_AAGR-N_obs3] - requests.exceptions.HTTPError: 404 Client Error: Not Found for url: https://oica.net/wp-content/uploads/CV-World-vehicles-in-use-2020.xlsx

As a workaround, the tests were marked XFAIL in that PR.

To resolve:

• Disable or remove code to retrieve upstream files that are no longer served.
• If possible, mirror the removed files on Zenodo, the TDC per se, or elsewhere.
• Add any newer files.