BUG: Disable check for oldstyle classes in python3 (np.info)

This fixes gh-2561.

During the test, nose raises a ResourceWarning. It has been fixed in newer versions of nose (see https://github.com/nose-devs/nose/issues/524). In the meantime I tried to disable it from within the test with 

    with warnings.catch_warnings():
        warnings.simplefilter("ignore", ResourceWarning)

but it didn't work as the warning is raised from outside the test.
Maybe the test itself is not really necessary ?