Cornice

- Rename default renderer from ``cornice`` to ``cornicejson`` (543)

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> We made the choice not to bump a major for this, because the default renderer name was never documented. If you ever were relying on the default renderer's name (`simplejson` for Cornice < 5.X, and `cornice` for 5.0.0 and 5.0.1), then I'm truely sorry for the breakage.

- Add setuptoools ``python_requires`` check

**Breaking Changes**

- Drop Python 2 support
- The default JSON renderer does not use ``simplejson.dumps()`` by default anymore, so the requirement has been dropped.

Please refer to [upgrading docs](https://cornice.readthedocs.io/en/latest/upgrading.html) for detailed migration instructions.

**Bug fixes**

- Fix support of schema as class instead of instance (523)

**Breaking changes**

- Check for ``MappingSchema`` instance in ``colander_*_validator`` (503)
(explicit error is raised when sequence schema is used for example)

**Bug fixes**

- Fix updating request.validated dict in ``colander_*_validator`` (503)

- Do not return content if response is ``204 No Content`` (fixes 521)