Remove depreacted pypi xml RPC api

[abs51295]

Right now, we use pypi xml RPC api to fetch a list of python packages from the registry: https://github.com/fabric8-analytics/fabric8-analytics-rudra/blob/master/rudra/utils/validation.py#L39. However, as seen on: https://warehouse.pypa.io/api-reference/xml-rpc/, it's deprecated. We should instead use https://pypi.org/simple/ to fetch the list of packages from pypi.