added OPENPYXL_KEEP_VBA environment variable to aid testing

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Added support for saving back to the same file when leep_vba=True

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merged latest repository

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Stub test for units module.

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Naive initial implementation.

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Handle value==0

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Use a range of values for tests. Some pretty weird results that need investigating.

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If using LXML set the default (empty) namespace to CONTYPES_NS so that the workbook is inspectable by MS tools.

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added tox environments for keep_vba and a fixture for datadir

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Add some background information on units used by Excel.

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Add some more documentation and correct tests for converting between pixels and points.

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Refactored tests with saner values and make them pass.

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Better test values, set accuracy to 100th of a degree.

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Cleanup some functions and add test values for colours.

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Merged in jbovey/openpyxl/1.9 (pull request #73)

A couple of fixes to the vba support

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Correct incorrectly converted test.

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Convert from nose style to pytest.

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Convert to pytest style and add check for expected value.

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Test module renamed as it is mainly to do with testing datetimes.

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rename : openpyxl/tests/test_number_format.py => openpyxl/tests/test_datetime.py
Tests refactored: test_datetime.py will only test the date_time module. Other tests moved to test_cell.py but some of them need fixing as they are dependent upon real worksheets.

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