Split from F841 following discussion in #8884.
Fixes#8884.
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## Summary
<!-- What's the purpose of the change? What does it do, and why? -->
Add a new rule for unused assignments in tuples. Remove similar behavior
from F841.
## Test Plan
Adapt F841 tests and move them over to the new rule.
<!-- How was it tested? -->
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Co-authored-by: Micha Reiser <micha@reiser.io>
c80678a1c0