Python 2.2 compatibility: Don't use UserDict.DictMixin.

Python 2.2 compatibility: Don't use UserDict.DictMixin.

UserDict.DictMixin was not included in Python 2.2, so we need to
define some auxiliary dictionary methods explicitly.

* cvs2svn: from __future__ import generators to enable the yield
  statement in Python 2.2.

  (AbstractDatabase): Don't inherit from DictMixin.

  (AbstractDatabase.__init__): Trivial changes to comment.

  (AbstractDatabase.__delitem__): Made comment more explicit, and
  changed it from a docstring to a normal comment.

  (AbstractDatabase.__iter__, AbstractDatabase.has_key,
  AbstractDatabase.__contains__, AbstractDatabase.iterkeys,
  AbstractDatabase.clear, AbstractDatabase.items,
  AbstractDatabase.values, AbstractDatabase.get): New methods
  (previously inherited from DictMixin).

git-svn-id: http://cvs2svn.tigris.org/svn/cvs2svn/trunk@1819 be7e6eca-30d4-0310-a8e5-ac0d63af7087