input: Write a function to create a web API using Python and Flask that returns the current date and time in the format "YYYY-MM-DD HH:MM:SS". The route to access this API should be `/datetime`. Use the Flask library and the `datetime` module for this purpose.
  assert get_datetime().strftime("%Y-%m-%d %H:%M:%S")==datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S")
output: 
  from flask import Flask
  import datetime
  
  app = Flask(__name__)
  
  @app.route('/datetime')
  def get_datetime():
      now = datetime.datetime.now()
      return now.strftime("%Y-%m-%d %H:%M:%S")
  