swh.lister.pypi: Move to pypi listing using the legacy api

The xmlrpc is marked as deprecated [1]. I started the implementation
with it against my better judgement (working now though). I move away
from it for a slightly simpler version using the legacy api [2].

I like it better because we use the same transport as the other
listers, providing for example our user-agent information to the
server.

[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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