input: Write a python function to modify the value of a specific key in a JSON string.
  assert modify_json_key_value('{ "name": "Alice", "role": "programmer", "department": "engineering" }', 'department', 'marketing') == '{"name":"Alice","role":"programmer","department":"marketing"}'
  assert modify_json_key_value('{ "name": "Bob", "role": "designer", "department": "art" }', 'role', 'lead designer') == '{"name":"Bob","role":"lead designer","department":"art"}'
  assert modify_json_key_value('{ "name": "Charlie", "role": "manager", "department": "finance" }', 'name', 'Charles') == '{"name":"Charles","role":"manager","department":"finance"}'
output: def modify_json_key_value(json_str, key, new_value):
      import json
      data = json.loads(json_str)
      data[key] = new_value
      return json.dumps(data, separators=(',', ':'))