gh-35127: Fix a very slow doctest in `sage/data_structures/stream.py`
This single test used to take ~ 200s.
Now it takes ~ 1.5s and I marked it long time.
This was introduced in 94219d42c (#34413).
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URL: https://github.com/sagemath/sage/pull/35127
Reported by: Gonzalo Tornaría
Reviewer(s): Gonzalo Tornaría, Martin Rubey, Travis Scrimshaw
