v0.3.0 - Marshmallow 3 Compatibility

This release adds support for Marshmallow 3, which introduced a number of breaking changes. We have retained support for Marshmallow 2, and intend to keep future releases backwards compatible as well.

We have also added explicit support for Python 3.7.

Finally, this release includes a bugfix by @timc13. Previously, if a custom json module had been set, that module was not used when serializing errors encountered during process_response. Now they are. Thanks, @timc13!