Fix ElementTree treewalker under cElementTree on Python 2.6.

This is effectively the treewalker equivalent of 3e50aad85d8113e7:

    Don't rely on the ElementTree Comment factory being the tag
    attribute on Comments.

    This is needed for xml.etree.cElementTree under 2.6 (and likely
    hence custom installs of cElementTree 1.2), where the tag property
    is equal to xml.etree.ElementTree.Comment (i.e., the pure Python
    version).