Extensionclass

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- Port the C extension to Python 3.

- Add support for Python 3.5 and 3.6.

- Drop support for Python 2.6, 3.2.

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- Fix calling of ``__class_init__`` hook by Python implementation.

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- Avoid wrapping ``__parent__`` in pure-Python version. Matches
change made to C version in afb8488. See issue 3.

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- Housekeeping changes only.

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- Added compatibility with Python 3.4.

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- Added compatibility with Python 3.2 and 3.3 using the Python reference
implementation.

- Add Python reference implementation. Used by default on PyPy.