Add a @cached_property implementation

[bluetech]

This is a useful utility to abstract the caching property idiom.

It is in compat.py since eventually it will be replaced by
functools.cached_property.

Fixes #6131.

[blueyed]

With regard to this I've noticed that we'd want to cache reportinfo() probably/maybe - any ideas how to do this? Should/could we wrap it in a property with a different name then? lru might also be an option, but seems bad with just a single item?!

[nicoddemus]

I think for methods we can implement a simple decorator that does something similar to cached_property, say cached_method. It should be even simpler than cached_property.

[bluetech]

lru might also be an option, but seems bad with just a single item?!

lru_cache is very fast in my experience (in Python terms...). However, it can't really be used on methods, it leaks self.

[RonnyPfannschmidt]

lets leave the method caching for later for now,
it can trivially be implemented as shim for a new private property if necessary

[bluetech]

Squashed and rebased -- will merge once it passes CI.

[bluetech]

Hmm, is adding a # pragma: no cover to the py38 branch the correct fix for the coverage CI failure? It seems wrong because it depends on the python version calculating the coverage. Or is coverage combined from multiple versions?

[blueyed]

@bluetech
It shows that we have no coverage for py38 (https://codecov.io/gh/pytest-dev/pytest/compare/710e3c40e0e613a06eef62f58b3f7db9761ee868...42a46ea78617b8e210636bc2f9d9bf06435b60fa/diff#D2-393). So either add it to the job (https://github.com/blueyed/pytest/blob/04f27d4eb469fb9c76fd2d100564a0f7d30028df/.travis.yml#L62-L63), or ignore it.
I recommend to not ignore uncovered branches - it's ok to merge things without 100% coverage, i.e. it is not a "CI failure", but only a hint.

[bluetech]

I recommend to not ignore uncovered branches - it's ok to merge things without 100% coverage, i.e. it is not a "CI failure", but only a hint.

Did you mean "not ignore" -> "ignore"?

In any case, I can't merge because it says "Required statuses must pass before merging". So I'll try to sort it out later.

[blueyed]

Did you mean "not ignore" -> "ignore"?

I've meant to not put "# pragma" comments there to ignore it with the report.
I.e. just ignore / accept that something is not covered (or rather not run with coverage reporting enabled to be more specific).

[blueyed]

I've removed the requirement on/for that check. /cc @nicoddemus