[MLIR] [Python] Fixed a long standing bug in the `PyRegionListIterator` (#188475) `PyRegionIterator` did not account for start index/step, so this commit removes it in favor of the sequence iterator provided by CPython. The previous attempt in #137232 bitrotted, so I decided to open a new PR. GitOrigin-RevId: 8afb4620d6b6d32341647ed31fa2f4273cff9ca3
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