Fix hasattr on Payload

__hasattr__ isn't a thing. hasattr calls getattr and looks for an AttributeError.

src/python/pants/base/payload.py

@@ -145,11 +145,11 @@ def mark_dirty(self):
       field.mark_dirty()
 
   def __getattr__(self, attr):
-    field = self._fields[attr]
+    try:
+      field = self._fields[attr]
+    except KeyError:
+      raise AttributeError(attr)
     if field is not None:
       return field.value
     else:
       return None
-
-  def __hasattr__(self, attr):
-    return attr in self._fields