bpo-34775: Return NotImplemented in PurePath division.

[aiudirog]

Return NotImplemented in PurePath.__truediv__ and PurePath.__rtruediv__ instead of raising a TypeError to allow custom compatible path types.

https://bugs.python.org/issue34775

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[jdemeyer]

It would be good to add a testcase.

[aiudirog]

I added a fairly simple test case with a basic custom class to test both sides of the operation and ensure a type error is only raised when both sides return NotImplemented. Please let me know if I should expand on it.

Thank you

[miss-islington]

Thanks @aiudirog for the PR, and @serhiy-storchaka for merging it 🌮🎉.. I'm working now to backport this PR to: 3.7.
🐍🍒⛏🤖

[miss-islington]

Thanks @aiudirog for the PR, and @serhiy-storchaka for merging it 🌮🎉.. I'm working now to backport this PR to: 3.8.
🐍🍒⛏🤖

[miss-islington]

I'm having trouble backporting to 3.7. Reason: 'Error 110 while writing to socket. Connection timed out.'. Please retry by removing and re-adding the needs backport to 3.7 label.

[bedevere-bot]

GH-15172 is a backport of this pull request to the 3.8 branch.

[ned-deily]

The 3.8 backport was languishing awaiting a merge; I manually merged it in time 3.8.0b4. I would prefer we not backport this to 3.7, given where we are in 3.7's lifecycle as the current behavior has been around for a long time.