Sprockets.mixins.mediatype

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- Return a "400 Bad Request" when an invalid Content-Type header is received
instead of failing with an internal server error

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- Import from collections.abc instead of collections (thanks nullsvm)

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- Do not log tracebacks when decoding the request body fails

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- Set Tornado PIN to >=5, <7
- Remove setuptools_scm

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- Add MessagePack dependencies to package extras (eg. `pip install sprockets.mixins.mediatype[msgpack]`)
- Update to minimum of ietfparse 1.5.1
- Drop support for Python < 3.7
- Drop support for Tornado < 5
- Remove deprecated :meth:`sprockets.mixins.mediatype.content.ContentSettings.from_application`.

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- Add support for Python 3.5 through 3.7
- Add support for Tornado < 6