Remove extra newline from N-Triples output

[tcmitchell]

Remove the extra newline generated by the N-Triples serializer and add
a unit test to verify that N-Triples output does not end in two
consecutive newlines.

Closes #1998

Summary of changes

This pull request removes the extra newline that the N-Triples serializers add. This pull request also includes a unit test to verify that the output from the N-Triples serializers ("nt" and "nt11") does not end in two consecutive newlines.

See issue #1998 for additional details and a reproducible example. This reproducible example is used in the added unit test.

This fix does not change any APIs so it should be backward compatible.

Checklist

• [ x] Checked that there aren't other open pull requests for
  the same change.
• [ x] Added tests for any changes that have a runtime impact.
• [ x] Checked that all tests and type checking passes.
• [ x] Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[aucampia]

LGMT, I don't really see a good reason to have multiple newlines either. Will see what others say though.

[coveralls]

Coverage remained the same at 88.556% when pulling 9842e1e on tcmitchell:1998-ntriples-newlines into 05dced2 on RDFLib:master.

[ghost]

LGTM, there's only one instance of trailing newline in the test suite (for i in *.nt; do tail -2 ${i} | grep -H -n ^$ ; done) and it's explicitly described as such:

cat nt-syntax-file-03.nt
#One comment, one empty line.

[aucampia]

Thanks for contributing @tcmitchell