Zulu

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- Add ``fold`` attribute support to ``Zulu``.
- Add ``zulu.to_seconds`` for converting units of time to total number of seconds.
- Add ``zulu.Timer`` class that can be used to track elapsed time (like a stopwatch) or as a countdown timer.

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- Drop support for Python 2.7.

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- Support Python 3.7.

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- Add ``Zulu.datetimetuple()``.
- Add ``Zulu.datetuple()``.
- Remove ``Zulu.__iter__`` method. (**breaking change**)
- Remove ``Delta.__iter__`` method. (**breaking change**)

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- Add Python 3.6 support.
- Add ``Delta.__iter__`` method that yields 2-element tuples like ``Zulu.__iter__``. Delta values are normalized into integer values distributed from the higher units to the lower units.
- Add ``Delta.__float__`` and ``Delta.__int__`` methods for converting to seconds.
- Add ``Zulu.__float__`` and ``Zulu.__int__`` methods for converting to epoch seconds.
- Fix issue in Python 3.6 where ``zulu.now()`` returned a naive datetime ``Zulu`` instance.
- Make ``Zulu.__iter__`` yield 2-element tuples containing ``(unit, value)`` like ``(('year', 2000), ('month', 12), ...)`` so it can be converted to a ``dict`` with proper keys easier. (**breaking change**)
- Remove previously deprecated ``zulu.delta()`` function. Use ``zulu.parse_delta()`` instead. (**breaking change**)
- Rename modules: (**breaking change**)

- ``zulu.datetime -> zulu.zulu``
- ``zulu.timedelta -> zulu.delta``

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- Provide fallback for the default value of ``locale`` in ``Delta.format()`` when a locale is not known via environment variables.