Using raise from in concurrency._next

[jamesbraza]

Summary

Minor style change using raise from in _next

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[Kludex]

Thanks, but that information is not used anywhere.