Add support for a Python decorator role

See previous discussion over at the CPython repo:

- https://github.com/python/cpython/issues/105812
- https://github.com/python/cpython/pull/105792#discussion_r1230584709
- https://github.com/python/cpython/pull/118743

The decorator (`:py:deco:`) role would render the function/method reference with a prefixed `@`.