bpo-23082: Better error message for PurePath.relative_to() from pathlib

[rotuna]

Changed the error message to be more informative and updated the docs accordingly.

https://bugs.python.org/issue23082

[the-knights-who-say-ni]

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[rotuna]

@zooba Could you please review this PR?

[zooba]

Sorry for the delay - I had it flagged to follow-up, but life.

Pinging me on GitHub doesn't work either, btw (too many notifications; too hard to filter them). If you need my attention, please post on the bug.

[idomic]

The PR looks good pinging on the bug's page

[miss-islington]

Thanks @rotuna for the PR, and @zooba for merging it 🌮🎉.. I'm working now to backport this PR to: 3.9.
🐍🍒⛏🤖 I'm not a witch! I'm not a witch!

[bedevere-bot]

GH-20395 is a backport of this pull request to the 3.9 branch.