WIP Node._repr_failure_py: use abspath with changed cwd

[blueyed]

Fixes #6428.

TODO:

• test
• changelog
• use relative path to invocation dir

[lhupfeldt]

I think relative to invocation dir is important to keep the error messages short and readable.

[nicoddemus]

Looks like a good start, thanks 👍

[lhupfeldt]

I think this should be merged now. Changing to use directory relative to invocation dir can be done later.

[blueyed]

@lhupfeldt there are 2 other TODOs anyway, and I'd rather have it finished when/while doing the test for it, instead of adjusting the test later again.
If you'd like to help you can give it a try.

[nicoddemus]

Superseded by #7220