Add offset argument to seek in io test

[bollwyvl]

while trying to package ipykernel for pypy on conda, we've run uncovered a small issue:

=================================== FAILURES ===================================
_________________________________ test_io_api __________________________________

    def test_io_api():
        """Test that wrapped stdout has the same API as a normal TextIO object"""
        session = Session()
        ctx = zmq.Context()
        pub = ctx.socket(zmq.PUB)
        thread = IOPubThread(pub)
        thread.start()
    
        stream = OutStream(session, thread, 'stdout')
    
        # cleanup unused zmq objects before we start testing
        thread.stop()
        thread.close()
        ctx.term()
    
        assert stream.errors is None
        assert not stream.isatty()
        with nt.assert_raises(io.UnsupportedOperation):
            stream.detach()
        with nt.assert_raises(io.UnsupportedOperation):
            next(stream)
        with nt.assert_raises(io.UnsupportedOperation):
            stream.read()
        with nt.assert_raises(io.UnsupportedOperation):
            stream.readline()
        with nt.assert_raises(io.UnsupportedOperation):
>           stream.seek()
E           TypeError: seek() missing 1 required positional argument: 'offset'

Docs suggest this is indeed the case: https://docs.python.org/3/library/io.html#io.IOBase.seek

The fix just gives it 0, but doesn't seem like it matters much, as it appears we're just checking that it subsequently fails.

Will skip the test on conda-forge, but it would be nice to have this for the next go-around... too bad i didn't find it earlier!

[blink1073]

Thanks!