Add some notes on improving IterableWorksheet

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Whitespace

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Allow test configuration to drive whether lxml is available or not.
Some tests must be skipped that make explicit use of lxml for validation.

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Mark tests which explicitly require lxml. Some of these might benefit from refactoring in the future as the tests do not just validate.

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Adjust nolxml environment to try and run all tests.

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Add pytest-flakes environment.

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Reorganise Python 3 specific names into one block.

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Cleaned up after running py.test —flakes.
Some tests do nothing so marked with xfail
Ignore assignments when SubElement() is used to create a child which itself has no children.

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Add a Python 2.6 / No lxml environment to catch API changes in xml.etree

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branch : 1.9