swh.lister.pypi: Add a pypi lister implementation using xmlprc api

Based solely on pypi's deprecated xmlrpc api [1]. No other way of listing
pypi.org is referenced (except for parsing an html page through a
legacy api [2])

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

[2] https://pypi.python.org/simple/

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