gl = gitlab.Gitlab('https://gitlab.******', private_token=os.environ['GITLAB_API_TOKEN'])
svc = gl.projects.get('test').services.get('slack')

slacksvc.properties['webhook'] = 'https://**************'
slacksvc.save()

send: b'PUT /api/v4/projects/33/services/slack HTTP/1.1\r\nHost: gitlab.****************\r\nUser-Agent: python-gitlab/2.5.0\r\nAccept-Encoding: gzip, deflate\r\nAccept: */*\r\nConnection: keep-alive\r\nPRIVATE-TOKEN: P**************f\r\nContent-type: application/json\r\nContent-Length: 32\r\n\r\n'
send: b'{"id": "slack"}'
reply: 'HTTP/1.1 400 Bad Request\r\n'
header: Server: nginx
header: Date: Sun, 13 Sep 2020 13:02:56 GMT
header: Content-Type: application/json
header: Content-Length: 30
header: Connection: keep-alive
header: Cache-Control: no-cache
header: Vary: Origin
header: X-Content-Type-Options: nosniff
header: X-Frame-Options: SAMEORIGIN
header: X-Request-Id: **********
header: X-Runtime: 0.017246
DEBUG:urllib3.connectionpool:http://gitlab.**************:80 "PUT /api/v4/projects/33/services/slack HTTP/1.1" 400 30
*** gitlab.exceptions.GitlabUpdateError: 400: webhook is missing

 gl = gitlab.Gitlab('https://gitlab.******', private_token=os.environ['GITLAB_API_TOKEN'])
 data = {'webhook': 'http://***************'}
 gl.http_put('/projects/{0}/services/slack'.format(gl.projects.get('test').id), post_data=data)