Migrate from xmlrpc to PyPIJSON

[gnikonorov]

PyPI has introduced a new JSON api. As a result, the xml-rpc api is being phased out.

SInce PyPI introduced rate limiting for the xml-rpc api ( see #59 and #60 ) it would make sense for us to migrate now since (a) the library is deprecated and (b) moving to JSON should provide performance gains

[nicoddemus]

Closed by #66.