F841: support fixing unused assignments in tuples by renaming variables

[sciyoshi]

Summary

A fairly common pattern which triggers F841 is unused variables from tuple assignments, e.g.:

user, created = User.objects.get_or_create(...)
      ^ F841: Local variable `created` is assigned to but never used

This error is currently not auto-fixable.

This PR adds support for fixing the error automatically by renaming the unused variable to have a leading underscore (i.e. _created) iff the dummy-variable-rgx setting would match it.

I considered using renamers::Renamer here, but because by the nature of the error there should be no references to it, that seemed like overkill. Also note that the fix might break by shadowing the new name if it is already used elsewhere in the scope. I left it as is because

1. the renamed variable matches the "unused" regex, so it should hopefully not already be used,
2. the fix is marked as unsafe so it should be reviewed manually anyways, and
3. I'm not actually sure how to check the scope for the new variable name 😅

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[charliermarsh]

Nice, this makes sense to me. Appreciate the clear rationale too!