Enhance http.py PEP8

gluon/http.py

@@ -116,7 +116,7 @@ class HTTP(Exception):
         for k, v in iteritems(headers):
             if isinstance(v, list):
                 rheaders += [(k, str(item)) for item in v]
-            elif not v is None:
+            elif v is not None:
                 rheaders.append((k, str(v)))
         responder(status, rheaders)
         if env.get('request_method', '') == 'HEAD':
@@ -148,7 +148,7 @@ class HTTP(Exception):
             web2py_error=self.headers.get('web2py_error'))
 
     def __str__(self):
-        "stringify me"
+        """stringify me"""
         return self.message