Use new Node.from_parent constructor when available

[takluyver]

Fixes compatibility with pytest >=6.

Closes gh-139

[takluyver]

I've copied the hasattr check from pytest's docs, so we support older versions of pytest. I don't know if from_parent was available before the alternative was deprecated, but the deprecation was in 5.4, released March 2020.

[mattwthompson]

Just stopping by to say this fixes the issues I've run into with pytest 6+ today (now that it's released and pulled in by default in a lot of builds)

[vidartf]

I'll do a patch release.

[vidartf]

Released as 0.9.6

[takluyver]

Thanks Vidar :-)

[vidartf]

Thanks for the PR ;)