chore(setup.py): enforce python version >=3.5 like in debian package

--HG--
branch : 0.1

setup.py

@@ -72,6 +72,7 @@ setup(
     classifiers=classifiers,
     packages=find_packages(exclude=['test']),
     install_requires=install_requires,
+    python_requires='>=3.5.0',
     include_package_data=True,
     entry_points={
         'cubicweb.cubes': [