Remove use_2to3 flag

The resolution of setuptools 2769 made any package using `use_2to3` to
fail its build.  This patch removes the flag, in support of outroducing
rdflib-jsonld.

The test suite is showing other follow-on patches will be necessary to
fix matters 2to3 had been quietly fixing along the way.  However, this
first patch does restore a working call to `pip install .` with
up-to-date setuptools.

Unit test results: This causes only the same five tests as were
previously failing to fail.

setuptools versions tested:
* 41.2.0
* 58.0.4

References:
* pypa/setuptools#2769
* RDFLib/rdflib#1405

Reported-by: Ralf Grubenmann <ralf.grubenmann@sdsc.ethz.ch>
Signed-off-by: Alex Nelson <alexander.nelson@nist.gov>

setup.py

@@ -77,7 +77,6 @@ def md2pypi(filename):
     url="https://github.com/RDFLib/rdflib-jsonld",
     license="BSD",
     packages=["rdflib_jsonld"],
-    use_2to3=True,
     zip_safe=False,
     platforms=["any"],
     classifiers=[