Zexceptions

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- Increase Python 3 compatibility

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- Add realm as an argument to unauthorized exceptions, its presence
causing a `WWW-Authenticate` header to be emitted.

- Set `location` header during `__init__` of redirect exceptions.

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- Drop support for Python 3.3, add support for Python 3.6.

- Use `str(self)` as detail if it is not set.

- Add a `setHeader` method to add a response header to an HTTPException.

- `upgradeException` now also supports finding an HTTPException class
with the same name as a non-HTTPException class.

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- Use `HTTPException.body_template` when title and detail are set.

- Add new title and detail attributes to HTTPException.

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- Add exception classes for all HTTP status codes.

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- Implement basic subset of Response features in HTTPException class.