input: Write a function to create a web API with Python and Flask to give the current date and time. The function should return the current date and time in the format "YYYY-MM-DD HH:MM:SS" when accessed via the '/datetime' route.
  
  assert app.route('/datetime') == "2023-10-05 14:30:45"
  assert app.route('/datetime') == "2023-10-15 10:20:30"
  assert app.route('/datetime') == "2023-10-25 18:45:12"
output: 
  @app.route('/datetime')
  def get_current_datetime():
      from datetime import datetime
      return datetime.now().strftime("%Y-%m-%d %H:%M:%S")
  