TST: skip tests on non-importable

COMPAT: provide CSV.close() method for compat
see my comment here: https://github.com/ContinuumIO/blaze/pull/893

I agree CSV objects are not opened. This is for compat with pretty much all backends AFAICT. (IOW this is an already established de-facto interface that seems to exist). This is not strictly necessary as the exception is caught in the referenced issue.