Update topic_tools relay_field to Python 3

[RyanDMott]

Solves

Issue 2167

Content

Implementing the "unicode" to "bytes" migration the author of Issue 2167 describes.

After this change, Python's 2to3 only suggests

• using list() to encase the result of a dictionary's items() before iterating over it (which is not useful) and
• deleting the import of print_function from future (which we might as well do).

Explanation

To address the second half of issue #2167, it is the OP's example which is at fault, not relay_field. The example attempts to write a string to a field ("data") whose type has changed from "unicode" to "bytes" in Noetic. Replacing the empty string with empty brackets [] fixes that example. The example in the file's help looks intact.

Testing

On my copy of Noetic the attached example launch files run when the edit in this PR is made. Both show the frame_id being transferred but the timestamp changed to one second.

test_relay_field.tar.gz

I have not tested beyond these two examples.

[peci1]

LGTM, but a test would help here. I think starting by copying relay.test and changing it to relay a field would be enough.