Checking which pypi packages are Py3k-only
import xmlrpc.client

# pypi language version classifiers
PY3 =  ["Programming Language :: Python :: 3"]


Why do you have a single string in a list?

PY2 =  ["Programming Language :: Python :: 2"]
PY27 = ["Programming Language :: Python :: 2.7"]
PY26 = ["Programming Language :: Python :: 2.6"]
PY25 = ["Programming Language :: Python :: 2.5"]
PY24 = ["Programming Language :: Python :: 2.4"]
PY23 = ["Programming Language :: Python :: 2.3"]


You should put all these strings in one list.

def main():
    client = xmlrpc.client.ServerProxy('http://pypi.python.org/pypi')

    # get module metadata
    py3names = [name[0] for name in client.browse(PY3)]
    py2names = [name[0] for name in client.browse(PY2)]
    py27names = [name[0] for name in client.browse(PY27)]
    py26names = [name[0] for name in client.browse(PY26)]
    py25names = [name[0] for name in client.browse(PY25)]
    py24names = [name[0] for name in client.browse(PY24)]
    py23names = [name[0] for name in client.browse(PY23)]


If you put the python 2.x versions in a one list, you should be able to fetch all this data
into one big list rather then all of these lists.

    cnt = 0


Don't uselessly abbreviate, spell out counter

    for py3name in py3names:
        if py3name not in py27names \
           and py3name not in py26names \
           and py3name not in py25names \
           and py3name not in py24names \
           and py3name not in py23names \
           and py3name not in py2names:
            cnt += 1


I'd do something like: python3_only = [name for name in py3name if py3name not in py2names]. Then I'd get the number of packages as a len of that.

    print("Python3-only packages: {}".format(cnt))

main()


Usually practice is to the call to main inside if __name__ == '__main__': so that it only gets run if this script is the main script.