Z3c-batching

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- 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.

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- Added a function to build a small neighbourhood list of the current batch,
from a large batch list. (extracted from z3c.table)

- Really fixed the bug with batches slicing