[bug] using custom JSON decoder

[KS-HTK]

From python 3.9 onward support for the encoding keyword has been dropped. While this is not an issue with geojson, it becomes an issue if a custom JSON Encoder/Decoder is used.

codec.py contains:

try:
    import simplejson as json
except ImportError:
    import json

this is an issue because the custom JSON Encoder/Decoder is subclassed of the python json encoder not the version from simplejson and since they are not compatible, if simplejson is present an issue will be thrown when simplejson tries to call the custom encoder with keywords it does not support.

This was opened as an issue with simplejson and they do not plan to be compatible with the standard json implementations so using it as a drop in replacement seems like a bad idea.
simplejson/simplejson#341