Below is a quick and dirty fix based on your code. It runs on Python 2.7. I hope it helps.
import urllib2
import StringIO
from lxml import etree
queryXML='''
<QUERIES>
<LANG>eng</LANG>
<AUTH>
<CLIENT>a_client_id</CLIENT>
<USER>a_user_id</USER>
</AUTH>
<QUERY CMD="ALBUM_SEARCH">
<TEXT TYPE="ARTIST">oasis</TEXT>
<TEXT TYPE="ALBUM_TITLE"></TEXT>
<TEXT TYPE="TRACK_TITLE">wonderwall</TEXT>
</QUERY>
</QUERIES>
'''.strip()
request = urllib2.Request("https://cxxxxxxx.web.cddbp.net/webapi/xml/1.0/", queryXML)
response = urllib2.urlopen(request)
response_page = response.read()
tree = etree.parse(StringIO.StringIO(response_page))
root = tree.getroot()
artist = root.find('.//ARTIST').text
print artist
Note that the response returns multiple results and this code only prints out the first one. Also error handling is missing here so please just use it just as a reference to get you started.