[pkg] Indicate that it is Python 3 only

It is usefull for download through PyPi, because it avoids to
download the new version that only support Python 3 if it is still
a Python 2 project.

cubicweb_rqlcontroller/__pkginfo__.py

@@ -23,5 +23,7 @@ classifiers = [
     'Environment :: Web Environment',
     'Framework :: CubicWeb',
     'Programming Language :: Python',
+    'Programming Language :: Python :: 3',
+    'Programming Language :: Python :: 3 :: Only',
     'Programming Language :: JavaScript',
 ]