[3.12] doc: Use super() in subclassed JSONEncoder examples (GH-115565) (GH-116047)

doc: Use super() in subclassed JSONEncoder examples (GH-115565)

Replace calls to `json.JSONEncoder.default(self, obj)`
by `super().default(obj)` within the examples of the documentation.
(cherry picked from commit 647053fed182066d3b8c934fb0bf52ee48ff3911)

Co-authored-by: Jan Max Meyer <jmm@phorward.de>