Python code

from osgeo import ogr, osr

driver = ogr.GetDriverByName('GeoJSON')
dataset = driver.Open('geojson.json')
layer = dataset.GetLayer()
feature = layer.GetNextFeature()
geom = feature.GetGeometryRef()

source = osr.SpatialReference()
source.ImportFromProj4("+proj=stere +lat_0=90 +lat_ts=60 +lon_0=264 +x_0=0 +y_0=0 +R=6371229 +units=m +no_defs")
target = osr.SpatialReference()
target.ImportFromEPSG(4326)
transform = osr.CreateCoordinateTransformation(source, target)
geom.Transform(transform)

result_json = geom.ExportToJson()

with open("result_python.json", "w") as outfile:
    outfile.write(result_json)