Update setup.py to ensure Python 2.6 dependencies are installed by pip.

setup.py

@@ -51,13 +51,16 @@ setup(
             'rqworker = rq.cli:worker',
         ],
     },
+    extras_require={
+        ':python_version=="2.6"': ['argparse', 'importlib'],
+    },
     classifiers=[
         # As from http://pypi.python.org/pypi?%3Aaction=list_classifiers
         #'Development Status :: 1 - Planning',
         #'Development Status :: 2 - Pre-Alpha',
         #'Development Status :: 3 - Alpha',
-        'Development Status :: 4 - Beta',
-        #'Development Status :: 5 - Production/Stable',
+        #'Development Status :: 4 - Beta',
+        'Development Status :: 5 - Production/Stable',
         #'Development Status :: 6 - Mature',
         #'Development Status :: 7 - Inactive',
         'Intended Audience :: Developers',