Btrees

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- Fix ``_p_changed`` when removing items from small pure-Python
BTrees/TreeSets and when adding items to small pure-Python Sets. See:
https://github.com/zopefoundation/BTrees/issues/13

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- Suppress testing 64-bit values in OLBTrees on 32 bit machines.
See: https://github.com/zopefoundation/BTrees/issues/9

- Fix ``_p_changed`` when adding items to small pure-Python
BTrees/TreeSets. See:
https://github.com/zopefoundation/BTrees/issues/11

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- Accomodate long values in pure-Python OLBTrees.

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- Add support for PyPy and PyPy3.

- Add support for Python 3.4.

- BTree subclasses can define ``max_leaf_size`` or ``max_internal_size``
to control maximum sizes for Bucket/Set and BTree/TreeSet nodes.

- Detect integer overflow on 32-bit machines correctly under Python 3.

- Update pure-Python and C trees / sets to accept explicit None to indicate
max / min value for ``minKey``, ``maxKey``. (PR 3)

- Update pure-Python trees / sets to accept explicit None to indicate
open ranges for ``keys``, ``values``, ``items``. (PR 3)

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- Fix value-based comparison for objects under Py3k: addresses invalid
merges of ``[OLI]OBTrees/OBuckets``.

- Ensure that pure-Python implementation of ``OOBTree.byValue`` matches
semantics (reversed-sort) of C implementation.

==================

- Issue 2: compilation error on 32-bit mode of OS/X.

- Test ``PURE_PYTHON`` environment variable support: if set, the C
extensions will not be built, imported, or tested.