gh-35265: Improve approximate order on getitem calls

    
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### 📚 Description

Calling `Stream_inexact.__getitem__(n)` for `n ==
self._approximate_order`, we can update `self._true_order` and
`self._approximate_order` as follows:

* if the result is non-zero, we now know that `n` is the true order
* otherwise, we know that `n+1` is a lower bound for the order.
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This fixes #35261, but it may not be the best fix, because it
potentially makes `__getitem__` slower.

Fixes #34556, which was the original issue.

### 📝 Checklist

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### ⌛ Dependencies
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URL: #35265
Reported by: Martin Rubey
Reviewer(s): Martin Rubey, Travis Scrimshaw