Decorator

Sangwoo Shim contributed a fix so that cythonized functions can be decorated.
Brian McFee pointed out an issue in the `decorator_apply` example and
Wim Glenn pointed out that the "fix" in version 5.1 broke
`decorator.contextmanager` even more. Both issues are now solved.

Added a function `decoratorx` using the `FunctionMaker` and thus
preserving the signature of `__code__` objects. Then fixed three small bugs:
- Sphinx was printing a few warnings when building the documentation, as
signaled by Tomasz Kłoczko
- functions decorated with `decorator.contextmanager` were one-shot,
as discovered by Alex Pizarro.
- `decorator.decorator` was not passing the kwsyntax argument.

Fixed a test breaking PyPy. Restored support for Sphinx.

Made the decorator module more robust when decorating builtin functions
lacking dunder attributes, like `dict.__setitem__`.

The decorator module was not passing correctly the defaults inside the
`*args` tuple, thanks to Dan Shult for the fix. Also fixed some mispellings
in the documentation and integrated codespell in the CI, thanks to
Christian Clauss.

The decorator module was not copying the __module__ attribute anymore.
Thanks to Nikolay Markov for the notice.