input: Write a python function to get Spotify access token using requests and json module.
  assert 'access_token' in get_spotify_token('YOUR_CLIENT_ID', 'YOUR_CLIENT_SECRET')
  assert isinstance(get_spotify_token('YOUR_CLIENT_ID', 'YOUR_CLIENT_SECRET')['access_token'], str)
  assert get_spotify_token('INVALID_CLIENT_ID', 'INVALID_CLIENT_SECRET').get('access_token') is None
output: def get_spotify_token(client_id, client_secret):
      import requests
      import json
  
      url = "https://accounts.spotify.com/api/token"
      headers = {
          "Content-Type": "application/x-www-form-urlencoded"
      }
      data = {
          "grant_type": "client_credentials"
      }
      auth = (client_id, client_secret)
  
      response = requests.post(url, headers=headers, data=data, auth=auth)
      if response.status_code == 200:
          return response.json()
      else:
          return {}