Replace cachedproperty with functools.cached_property (>= 3.8)

[cdce8p]

I thought about this PR recently again. Typing cachedproperty might not work, but it can be replaced with functools.cached_property. We only need to sys guard it for < 3.8. This should work

if sys.version_info >= (3, 8):
    from functools import cached_property
else:
    from astroid.decorators import cachedproperty as cached_property

Additionally, the deprecation warning can be limited to >= 3.8.

Originally posted by @cdce8p in #1243 (comment)

[DanielNoord]

@cdce8p Just thinking out loud: can we also use a type guard to define cached_property? Would mypy pick up on that?

[cdce8p]

@cdce8p Just thinking out loud: can we also use a type guard to define cached_property? Would mypy pick up on that?

Not completely sure what you want to do with that.

On other thing, I just saw that we don't set the python-version for mypy. If we do that, we probably need to do some more workarounds to tell mypy cachedproperty is equal to cached_property. Adding TYPE_CHECKING could work

if sys.version_info >= (3, 8) or TYPE_CHECKING:
    from functools import cached_property
else:
    from astroid.decorators import cachedproperty as cached_property