Use relative import in nvmet/__init__.py

This is supported by Python 2.7 and later.

Signed-off-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

nvmet/__init__.py

-from nvme import Root, Subsystem, Namespace
+from .nvme import Root, Subsystem, Namespace