Z3c-currency

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- Add support for Python 3.5, 3.8 and 3.9.

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- Python 3.6 and 3.7 support. Drop Python 2.6.

- Tests now run with `python setup.py test`

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- Standardize namespace __init__

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- Added new precision value "sub-cents" (``interfaces.SUBCENTS``), which
allows precision beyond pennies, which is needed for financial and other
business applications.

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- Updated Trove classifiers.

- Moved code to GitHub.

- Changed validation to raise custom validation errors, since the upstream
code looks at the doc string of the exception instead of the first argument.

- Improved converter to

* Ensure proper precision of decimal after initial parsing.

* Format the value to the proper precision.

- Changed "precision" field in `ICurrency` interface to be a choice, so that
UIs generate nicely.

- Cleaned up code a little bit.

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- Initial Release

* Implementation of ``Currency`` field supporting precision and a unit.

* Implementation of data converter.