Your input appears to be a sequence of Python objects; it certainly is not valid a JSON document.

If you have a list of Python dictionaries, then all you have to do is dump each entry into a file separately, followed by a newline:

import json

with open('output.jsonl', 'w') as outfile:
    for entry in JSON_file:
        json.dump(entry, outfile)
        outfile.write('\n')

The default configuration for the json module is to output JSON without newlines embedded.

Assuming your A, B and C names are really strings, that would produce:

{"index": 1, "met": "1043205", "no": "A"}
{"index": 2, "met": "000031043206", "no": "B"}
{"index": 3, "met": "0031043207", "no": "C"}

If you started with a JSON document containing a list of entries, just parse that document first with json.load()/json.loads().

I use a trick to do what I want, to rewrite all the objects into a new line. I write what I want to keep into a newfile.

with open('verbes_lowercase.json','r',encoding='utf-8-sig') as json_data:
    data=json.load(json_data)
    with open("verbes.json",'w',encoding="utf-8-sig") as file:
        file.write("[")
        length=len(data)
        for k in range(0,length):
            del data[k]["attribute1"]
            if (k!=length-1):
                 file.write(json.dumps(data[k], ensure_ascii=False)+",\n")
            else:
                file.write(json.dumps(data[length-1], ensure_ascii=False)+"]")