same method that implements __setitem__ also implements __delitem__.
Also, there were several good use cases (removing items from a queue
and implementing Forth style stack ops).
* The default __reversed__ performed badly, so reintroduced a custom
reverse iterator.
* Added length transparency to improve speed with map(), list(), etc.
__getitem__() and __setitem__().
Simplifies the API, reduces the code size, adds flexibility, and makes
deques work with bisect.bisect(), random.shuffle(), and random.sample().
* Add doctests for the examples in the library reference.
* Add two methods, left() and right(), modeled after deques in C++ STL.
* Apply the new method to asynchat.py.
* Add comparison operators to make deques more substitutable for lists.
* Replace the LookupErrors with IndexErrors to more closely match lists.
Raymond Hettinger
Neal Norwitz