Incompatibility with flask 2.3

[totycro]

The relase notes for flask 2.3 contain this:

json_encoder and json_decoder attributes on app and blueprint, and the corresponding json.JSONEncoder and JSONDecoder classes, are removed.

There is some more information in the docs for 2.2 about JSONEncoder.

Currently JSONEncoder is used here in flasgger, leading to this import error:

my_api/__init__.py:9: in <module>
    from flasgger import Swagger
/usr/local/lib/python3.10/site-packages/flasgger/__init__.py:10: in <module>
    from .base import Swagger, Flasgger, NO_SANITIZER, BR_SANITIZER, MK_SANITIZER, LazyJSONEncoder  # noqa
/usr/local/lib/python3.10/site-packages/flasgger/base.py:28: in <module>
    from flask.json import JSONEncoder
E   ImportError: cannot import name 'JSONEncoder' from 'flask.json' (/usr/local/lib/python3.10/site-packages/flask/json/__init__.py)

I think the fix would be to port the serialization behavior to JSONProvider.

There is some information in the PR that introduced JSONProvider about why they changed this.