Enable pylint to support decorator.contextmanager

Using decorator.contextmanager trigger the pylint error E1129 (not-context-manager).

Pylint has a configuration option for this:

    # List of decorators that produce context managers, such as
    # contextlib.contextmanager. Add to this list to register other decorators that
    # produce valid context managers.
    contextmanager-decorators=contextlib.contextmanager,decorator.contextmanager

But the current implementation of decorator.contextmanager prevents this from working.

Changing contextmanager to be a simple function fixes the issue.