As you can see in my already posted Code, I already use one of the right scopes. I also tried the other two scopes, still Errorcode 403. I also checked the other URLs in Google OAuth Playground, they are also correct.

I know the playlistId and also the videoId. The are definitive correct and not the problem.

Yes, it is my own playlist. On the Google site it is possible to add a video to the playlist. But the site is not the same as the code used.

I tried your approach, but with the same result: forbidden (403) playlistItemsNotAccessible.

I found the scope in this site: https://developers.google.com/youtube/v3/docs/playlistItems/insert
There is also some Python Code, but it's also not working in Pythonista.

Finally I found some time to try your approach and voila! it starts to work! Now I am able to connect to the server and get the tokens, everything works until I want to add the video to the playlist. Then I get a

forbidden (403)	playlistItemsNotAccessible

I will post my (modified) code here once again:

def playlistItem_insert(videoId, key):
with open(CLIENT_SECRETS_FILE, 'r') as file_obj:
   client_credentials = json.load(file_obj)
private_key_pkcs8_pem = client_credentials['private_key']
signer = _pure_python_crypt.RsaSigner.from_string(private_key_pkcs8_pem)

header = {'typ': 'JWT', 'alg': 'RS256'}
now = int(time.time())
payload = {
       'aud': 'https://www.googleapis.com/oauth2/v4/token',
       'scope': 'https://www.googleapis.com/auth/youtube.force-ssl',
       'iat': now,
       'exp': now + 3600, # 1 hour in seconds
       'iss': service_account_email_address
}
segments = [
       _helpers._urlsafe_b64encode(_helpers._json_encode(header)),
       _helpers._urlsafe_b64encode(_helpers._json_encode(payload)),
]
signing_input = b'.'.join(segments)
signature = signer.sign(signing_input)
segments.append(_helpers._urlsafe_b64encode(signature))
claim = b'.'.join(segments)

post_data = {
       'grant_type':'urn:ietf:params:oauth:grant-type:jwt-bearer',
       'assertion':claim
}
req = requests.post('https://www.googleapis.com/oauth2/v4/token', post_data)
resp = req.json()
access_token = resp['access_token']
auth_header = {
       'Authorization':'Bearer ' + access_token
}

# Use the header in service requests, e.g.
d={'snippet': { 
'playlistId': PLAYLIST_ID, 'resourceId': { 
'kind': 'youtube#video', 
'videoId': videoId}
}
}
req = requests.post('https://www.googleapis.com/youtube/v3/playlistItems?part=snippet&key='+key, json=d, headers = auth_header)
req.raise_for_status()
return req.status_code

Parameters are all available. The problem occurs in line req = requests.post(...)
Maybe someone has better eyes than me...

I wrote a little script that gets me a video-Id from Youtube that I want to add to a playlist. Now I tried to execute the following:

#add to playlist
url_post='https://www.googleapis.com/youtube/v3/playlistItems?part=snippet&key='+key
data={"snippet": {
    "playlistId": playListId,
    "resourceId": {
        "videoId": videoId,
        "kind": "youtube#video"
        }
    }
}
data_json=json.dumps(data)
response=requests.post(url_post, data=data_json)

I got a 401-Unauthorized and read the following link https://developers.google.com/youtube/v3/docs/playlistItems/insert where is mentioned, that I have to authenticate with OAuth. So I created some OAuth 2.0-Client-IDs in https://console.developers.google.com.
But now I don't have any clue how to use OAuth to get this working in Pythonista.
So how can I authenticate at Youtube to add a Video to my playlist?

Ok thanks for the workaround! I will try it.

I wrote a little script in Pythonista that reads the song from my favorite radio station. This combined with a (Apple) Workflow adds this title to a playlist.
Now my problem is, that I want to start the Workflow script as Today Widget. But if I do so, Pythonista is always opened and the the Workflow script stops until I go back from Pythonista.
Is there any way to run Pythonista-Script in background when executing inside a Workflow? It's not really helpful when I have to interact each time.
Thanks already in advance!