cached_property from python3.8 functools is not considered property

[gaborbernat]

It's because this property does not pass the check of isinstance(a, property) - see https://bugs.python.org/issue41523

https://github.com/sphinx-doc/sphinx/blob/3.x/sphinx/util/inspect.py#L305-L307 needs to change to:

from functools import cached_property

def isproperty(obj: Any) -> bool:
    """Check if the object is property."""	
    return isinstance(obj, (property, cached_property))

And guarded to be 3.8+.