use a try / except to check if argparse is a needed dependency

setup.py

@@ -6,7 +6,10 @@ import platform
 VERSION = "3.0" # find a better way to do so.
 
 requires = ['feedgenerator', 'jinja2', 'pygments', 'docutils', 'pytz']
-if sys.version_info < (2,7):
+
+try:
+    import argparse
+except ImportError:
     requires.append('argparse')
 
 scripts = ['bin/pelican', 'tools/pelican-themes', 'tools/pelican-import', 'tools/pelican-quickstart']