Xlutils

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- Take advantage of "ragged rows" optimisation in xlrd 0.7.3

- Add support for PANE records to :mod:`xlutils.copy`, which means that zoom
factors are now copied.

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- Removal of references in the `finish` methods of several filters,
easing memory usage in large filtering runs

- Speed optimisations for :class:`~xlutils.filter.BaseFilter`, bringing those
benefits to all subclasses.

- Memory usage reduction when using :class:`~xlutils.filter.MemoryLogger`

----------------------

- Add sheet density information and onesheet option to
:mod:`xlutils.margins`.

- Reduced the memory footprint of :class:`~xlutils.filter.ColumnTrimmer` at the
expense of speed.

- Fixed incorrect warnings about boolean cells in
:class:`~xlutils.filter.ErrorFilter`. :mod:`xlwt` has always supported boolean
cells.

- :class:`~xlutils.filter.BaseReader` now opens workbooks with ``on_demand = True``

- Added support for :mod:`xlrd` Books opened with ``on_demand`` as ``True`` passed to
:func:`~xlrd.open_workbook`.

- Fixed bug when copying error cells.

- Requires the latest versions of :mod:`xlrd` (0.7.1) and :mod:`xlwt` (0.7.2).

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- Made installation work when `setuptools` isn't present.

- Made `errorhandler` an optional dependency.

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- In xlutils.styles, handle there case where two names were mapped to the
same xfi, but the first one was empty.

-------------------

- fix bug that cause BaseWriter to raise exceptions when handling
source workbooks opened by xlrd 0.7.0 and above where
formatting_info had been passed as False

- add xlutils.copy