Use unicode chars instead of escape sequences in json encoder output and bump simplejson version

[mdsakalu]

Fixes #87 and Fixes #62

Make the Encoder class in lambda_runtime_marshaller use actual unicode characters in the encoded output instead of unicode escape sequences. This matters because unicode escape sequences contribute more bytes to the response size.

Also bump simplejson version for Python 3.9 compatibility.

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[briensea]

Thanks for contributing!