Here's another patch which addsd iter to dbm and gdbm.

Note that dbm and gdbm C API is a little different.
gdbm_nextkey requires key for its argument, dbm_nextkey don't.
So I had to use for gdbm an static variable that points to the current
position.
Now iterator in gdbm and dbm works differently.

>>> import dbm
>>> d = dbm.open('foo', 'n')
>>> d['k1'] = 'v1';d['k2'] = 'v2';
>>> for i in d: print i; break
... 
k1
>>> for i in d: print i
... 
k2
>>> for i in d: print i
... 


>>> import gdbm
>>> gd = gdbm.open('foo.gdbm', 'n')
>>> gd['k1'] = 'v1';gd['k2'] = 'v2';
>>> for i in gd: print i; break
... 
k2
>>> for i in gd: print i
for i in gd: print i
... 
k1
>>> for i in gd: print i
... 
k2
k1