Z3c.batching

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- Nothing changed yet.

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- Add support for Python 3.10, 3.11.

- Drop support for Python 2.7, 3.5, 3.6.

- Add support for Python 3.8 and 3.9.

- Drop support for Python 3.4.

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- Add support for Python 3.6, 3.7 and PyPy3.

- Drop support for Python 2.6 and 3.3.

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- Support Python 3.3 through 3.5.

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- Standardize namespace __init__

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- New feature: Subset batch.
Sometimes (for performance reasons), even though the user needs
a batched UI, we want to limit the computation to the
subset of values actually shown to the user.

- Register `batch.Batch` as named (``"z3c.batching.batch"``) factory.