def virtualenv_requirements(version_info=sys.version_info):
    """
    Generate Python version specific virtualenv package requirements.

    The requirements are returned as setuptools/pip compatible requirement
    specification strings.

    """
    if version_info < (2, 5):
        # 'virtualenv' release 1.8 dropped Python 2.4.x compatibility.
        yield requirement_spec("virtualenv",
            ("<", lowest_version_string_with_prefix("1.8")))
    elif version_info < (2, 6):
        # 'virtualenv' release 1.10 dropped Python 2.5.x compatibility.
        yield requirement_spec("virtualenv",
            ("<", lowest_version_string_with_prefix("1.10")))
    else:
        yield requirement_spec("virtualenv")