Transaction

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- Drop support for Python 2.6 and 3.2.

- Add support for Python 3.5.

- Added APIs for interogating and clearing internal state to support
client tests.

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- Use the standard ``valuerefs()`` method rather than relying on
implementation details of ``WeakValueDictionary`` in ``WeakSet``.

- Add support for PyPy3.

- Require 100% branch coverage (in addition to 100% statement coverage).

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- Add support for Python 3.4.

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- Released in error as 1.4.3.

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- Document that values returned by ``sortKey`` must be strings, in order
to guarantee total ordering.

- Fix occasional RuntimeError: dictionary changed size during iteration errors
in transaction.weakset on Python 3.

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- Updated Trove classifiers.