Flapison

* Allow providing content parsers and renderers as class variables (https://github.com/TMiguelT/flapison/pull/4)

* Allow providing the `*_schema_kwargs` family as a function (https://github.com/miLibris/flask-rest-jsonapi/pull/179)

* Allow custom content types (https://github.com/miLibris/flask-rest-jsonapi/pull/171)
* Bump Marshmallow to Python 3, and remove Python 2 support (https://github.com/miLibris/flask-rest-jsonapi/pull/172)
* Automatically filter resource lists by their parents (https://github.com/miLibris/flask-rest-jsonapi/pull/177)
* Assign the correct endpoint string to each Resource instance (https://github.com/miLibris/flask-rest-jsonapi/pull/178)