Kotti-image

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- Use ``rfc_6266_parser`` instead of unmaintained ``rfc_6266`` package.
- Fix tests for Kotti >= 2.0.0 / Python 3.x.

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- No functional changes. Maintainance release for Kotti 1.x.
- Integrate filedepot tween / filter test from Kotti < 2.0.0.

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- Correctly declare dependencies.

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- Include (empty) ``alembic/versions`` directory in distribution to avoid
warnings.

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- Use rfc6266 and unidecode for content disposition header.

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- Fix broken packaging.