Authencoding

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- Add support for Python 3.6, 3.7 and PyPy3.

- Drop support for Python 2.6, 3.3 and 3.4.

- Add ``BCRYPTHashingScheme``, optionally available if package is
installed with the `bcrypt` extra.

- Accept bytes as input to ``AuthEncoding.is_encrypted``.

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- Supporting Python 3.3 up to 3.5 and PyPy2.

- Added ``SHA256DigestScheme``.

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- Extracted from ``AccessControl 3.0.11``