unused variable regression 0.0.26->0.0.27

### Summary



```
def buy_sell_once(prices: list[float]) -> float:
    assert len(prices) > 1
    best_buy, best_so_far = prices[0], 0.0
    for i in range(1, len(prices)):
        best_so_far = max(best_so_far, prices[i] - best_buy)
        best_buy = min(best_buy, prices[i])
    return best_so_far

```


with 0.0.27, we get warning: "`best_buy` is unused" in line 6.

### Version

_No response_