use argparse dependence only if python<2.7

setup.py

@@ -18,10 +18,12 @@ install_requires = [
           'psutil>=3.0.0',
           'lxml',
           'python-magic',
-          'argparse',
           'erp5.util'
           ]
 
+if sys.version_info < (2, 7):
+  install_requires.append('argparse')
+
 setup(name='cloudooo',
       version=version,
       description="XML-RPC document conversion server",