Fix brokenness when gdbm is the selected anydbm module.
Also, do some AbstractDatabase refactoring.
* cvs2svn: import UserDict.
(AbstractDatabase): Derive from UserDict.DictMixin.
(AbstractDatabase.__init__): Do not directly copy __delitem__ from anydbm
object. Copy several methods from the internal anydbm object.
(AbstractDatabase.__delitem__): Provide wrapper for anydbm types which do
not have the method.
(AbstractDatabase.get): Delete, DictMixin will provide.
(SymbolingsReader.__init__): Use the increased compliance of Database with
the mapping interface to avoid the need to poke into its internals.
git-svn-id: http://cvs2svn.tigris.org/svn/cvs2svn/trunk@1745 be7e6eca-30d4-0310-a8e5-ac0d63af7087