Acquisition

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- Fix C code under Python 3.4, with missing Py_XSETREF.

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- Enable the C extension under Python 3.

- Drop support for Python 3.3.

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- Make tests compatible with ExtensionClass 4.2.0.

- Drop support for Python 2.6 and 3.2.

- Add support for Python 3.5 and 3.6.

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- Make the pure-Python Acquirer objects cooperatively use the
superclass ``__getattribute__`` method, like the C implementation.
See https://github.com/zopefoundation/Acquisition/issues/7.

- The pure-Python implicit acquisition wrapper allows wrapped objects
to use ``object.__getattribute__(self, name)``. This differs from
the C implementation, but is important for compatibility with the
pure-Python versions of libraries like ``persistent``. See
https://github.com/zopefoundation/Acquisition/issues/9.

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- Correct several dangling pointer uses in the C extension,
potentially fixing a few interpreter crashes. See
https://github.com/zopefoundation/Acquisition/issues/5.

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- Add support for PyPy, PyPy3, and Python 3.2, 3.3, and 3.4.