During the patching work of issue8634, I found several problems about the module dbm.gnu and dbm.ndbm, I feel it's better to address them on a separate issue.

1. As issue8634 said, the dbm.gnu, dbm.ndbm and dbm.dumb should have the similar interface, since they are intended to use the general dbm module as a common interface. If some methods only appears in one or two of them, like the "get()" method, it will be not convenient for users. Making all the three ones follow the collections.MutableMapping interface could be a better choice. Since the dbm.dumb is already collections.MutableMapping, I implemented missing methods of collections.MutableMapping in dbm.gnu and dbm.ndbm, and register the dbm object type in dbm.gnu and dbm.ndbm to the ABC. The missing methods mainly include get(), values(), items(), pop(), popitem(), clear()

2. I fix the dbm_contains() function which implement the "in" operator to accept all buffer object, just like the dbm_subscript() fuction which implment the "[]" slice operator. I feel it's wearied that if "dbm['a']" yields the expected result but at the same time "'a' in dbm" raises TypeError. 

3. The type of dbm object in dbm.gnu is not iterable, and there is no way to iterate on it sufficiently because the only way we can iterate over it is to get all the keys first using keys() and then iter on the keys list. So I implemented a iterator type for dbm.gnu. Besides the dbm object in dbm.ndbm is also not iterable, and I implemented its tp_iter by get an iterator from its keys list instead of implementing a "real" iterator since the ndbm c api for get all the keys one by one is thread specific and I could not found a way to implement a real iterator to avoid the problem which occurred in the case we get tow iterators from one db object in the same thread and iterate over both at the same time.

The patch contains also unittest and doc.