Patch to add a ContextDecorator class to contextlib. This allows context managers that inherit from ContextDecorator (including using it as a mixin) to be used as decorators as well as context managers.

Context managers inheriting from ContextDecorator still have to implement __enter__ and __exit__ as normal. As the decorator behaviour is implemented using a with statement __exit__ retains its optional exception handling even when used as a decorator.

In addition contextlib.GeneratorContextManager, used to implement contextlib.contextmanager, inherits from ContextDecorator. Context managers created with contextlib.contextmanager can therefore be used as decorators too.