Replaced lxml with xml.etree.ElementTree

Both flake8 related failures are due to the try: except: importing of xmlrpclib
    pep438/core.py:6:1: PY3K (FixImports) import xmlrpclib; -> import xmlrpc.client;
    pep438/core.py:37:1: PY3K (FixImports) xmlrpclib.ServerProxy('https://pypi.python.org/pypi'); -> xmlrpc.client.ServerProxy('https://pypi.python.org/pypi');