Simple geojson builder from a python dictionary, it works.

DictToGeoJson.py

import json

class Core:
    
    def json_gen(self, items_list: list):
        self.geojson = {}
        self.geojson['type'] = 'FeatureCollection'
        self.features = []

        for item in items_list:
            self.feature = {}
            self.properties = {}
            self.geometry = {}
            self.coordinates = []

            self.coordinates.append(item['lon'])
            self.coordinates.append(item['lat'])
            self.geometry['type'] = 'Point'
            self.geometry['coordinates'] = self.coordinates
            self.feature['type'] = 'Feature'
            self.feature['properties'] = self.properties
            self.feature['geometry'] = self.geometry
            self.features.append(self.feature)
           
        self.geojson['features'] = self.features
        
        with open('a.txt', 'w') as f:
            json.dump(self.geojson, f)