decrease DictRow memory usage

Use "__slots__ = ('_index',)" to reserve single attribute slot for _index.
Otherwise a per-row dict is created to store object attributes...

The only danger of doing it is that if somebody uses plain DictRow and
adds new attrubutes into it, it will get error:

  row.foo = x

I'm really dubious if anybody does this.  NB, this does not apply if
somebody has subclassed DictRow, then the parent class' __slots__
is ignored.

Attached patch does the same for RealDictRow too, although I personally
do not use it.  But it seems good idea to do it there too.

On older python versions that do not support __slots__ it should be
taken as random class attribute, so should be no problem.

-- 
marko

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diff --git a/lib/extras.py b/lib/extras.py
index 4a845ba..d6337ac 100644
--- a/lib/extras.py
+++ b/lib/extras.py
@@ -107,6 +107,8 @@ class DictCursor(DictCursorBase):
 class DictRow(list):
     """A row object that allow by-colun-name access to data."""
 
+    __slots__ = ('_index',)
+
     def __init__(self, cursor):
         self._index = cursor.index
         self[:] = [None] * len(cursor.description)
@@ -191,6 +193,9 @@ class RealDictCursor(DictCursorBase):
             self._query_executed = 0           
 
 class RealDictRow(dict):
+
+    __slots__ = ('_column_mapping',)
+
     def __init__(self, cursor):
         dict.__init__(self)
         self._column_mapping = cursor.column_mapping