All the dbm.* modules currently have different interfaces, and different
levels of supporting the Python3-style dictionary interface -- while the
docs claim they all have (most of) the dict interface.

For example, both dbm.gnu and dbm.ndbm only have keys() methods, and
they return a list.  Etc. for other dict-style methods.

So, either we remove the claim that they have a dict-style interface
beyond __*item__() and keys(), or we do something about it.