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* Added *extra_css* and *extra_js* config options to ReactJSONSchemaField class
that allows injecting extra assets required for the filed
* Added *on_render* config option to ReactJSONSchemaField class that adds an ability to
configure dynamic *schema* and *ui_schema* based on object fields
* Updated some *react* amd *react-dom* dependencies
* Added support for Python 3.9 and Django 2.2, 3.0 and 3.1 in testing environment