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

@@ -68,14 +68,13 @@ def md2pypi(filename):
 setup(
     name=name,
     version=version,
-    description='rdflib extension adding JSON-LD parser and serializer',
-    long_description=md2pypi('README.md'),
-    maintainer='RDFLib Team',
-    maintainer_email='rdflib-dev@google.com',
-    url='https://github.com/RDFLib/rdflib-jsonld',
-    license='BSD',
-    packages=['rdflib_jsonld'],
-    use_2to3=True,
+    description="rdflib extension adding JSON-LD parser and serializer",
+    long_description=md2pypi("README.md"),
+    maintainer="RDFLib Team",
+    maintainer_email="rdflib-dev@google.com",
+    url="https://github.com/RDFLib/rdflib-jsonld",
+    license="BSD",
+    packages=["rdflib_jsonld"],
     zip_safe=False,
     platforms=['any'],
     classifiers=[