Specify reason for Python 2.7 requirement

While ElementTree is in Python 2.6, we need the expanded support
for xpath which was introduced in Python 2.7.

As discussed in #7264, it might be possible to install
`elementtree-1.3a3-20070912` for Python 2.6, but this is yet untested.
Merge pull request #7513 from holmboe/skipif-version_info

Specify reason for Python 2.7 requirement