Make astropy.vo compatible with Python 2 and 3 without requiring 2to3.

As mentioned in #2033 .

All `astropy.vo` tests passed in Python 2.7, 3.2, 3.3, and 3.4 using the following commands:

    import astropy
    astropy.test('vo', remote_data=True)

Travis also passed (without remote data) except for a seemingly unrelated error for Python 2.7 in https://travis-ci.org/pllim/astropy/jobs/18151584 .