Zulu

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- Add support for Python 3.12
- Replace usage of deprecated ``datetime.utcnow`` and ``datetime.utcfromtimestamp``.

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- Drop support for Python 3.4 and 3.5.

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- Fix compatibility issue with Python 3.4 and 3.5 by replacing f-string usage with string-format.

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- Add Python 3.9 support.
- Fix bug in ``Zulu.time_from``, ``Zulu.time_to``, ``Zulu.time_from_now``, and ``Zulu.time_to_now`` where keyword arguments weren't passed to underlying ``Delta.format`` call.
- Fix bug in ``Zulu.format`` where "YY" and "YYYY" format patterns would return the year in "Week of Year" based calendars instead of the regular calendar year.

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- Add Python 3.8 support.
- Add ``'week'`` option to ``Zulu.start_of``, ``Zulu.end_of``, ``Zulu.span``, and ``Zulu.span_range``. Thanks ThomasChiroux_!
- Fix bug in ``Zulu.astimezone`` in Python 3.8 due to change in return type from ``super().asdatetime``. In Python<=3.7, ``super().asdatetime`` returned as instance of ``datetime``, but in Python 3.8 another instance of ``Zulu`` was returned instead. ``Zulu.astimezone`` will now return a ``datetime`` instance in Python 3.8.

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- Remove unused parameter in ``zulu.Timer.__init__()``.