input: Write a function to create a Flask web API that returns the current date and time in the format "YYYY-MM-DD HH:MM:SS". The endpoint should be `/datetime`.
  from flask import Flask
  import datetime
  
  app = Flask(__name__)
  
  @app.route('/datetime')
  def get_datetime():
      return datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S")
  
  if __name__ == '__main__':
      app.run()
  
  assert get_datetime()=="2023-10-05 14:30:45"
  assert get_datetime()=="2023-10-05 14:30:46"
  assert get_datetime()=="2023-10-05 14:30:47"
  assert get_datetime()=="2023-10-05 14:30:48"
  assert get_datetime()=="2023-10-05 14:30:49"
output: 
  def get_datetime():
      return datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S")
  