swh.lister.pypi: Use pypi's legacy html based api to list packages

The xmlrpc is marked as deprecated [1]. Even if it's not now, the
legacy api is not marked as deprecated. So moving towards this one
sounds more reasonable [2].

[1] https://warehouse.readthedocs.io/api-reference/xml-rpc/#pypi-s-xml-rpc-methods

[2] https://warehouse.readthedocs.io/api-reference/legacy/#simple-project-api

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