swh.lister.pypi: Use xmlrpc api to list pypi's origins
Following discussion with team, the xmlrpc api is not deprecated
today. It will not disappear soon.
Also, as:
- parsing the legacy html api [1] is considered bad practice
- discussions exist to create equivalent apis to their deprecated/legacy apis [1] [2]
We chose to implement the xmlrpc one.
[1] https://warehouse.readthedocs.io/api-reference/legacy/#simple-project-api
[2] https://github.com/pypa/warehouse/issues/284
[3] https://github.com/pypa/warehouse/pull/4078
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