input: Write a function to create a web API using Flask in Python that returns the current date and time in the format "YYYY-MM-DD HH:MM:SS". The function should define a Flask application with a route '/datetime' that calls a method to display the current date and time. The method should be able to run the Flask application when executed.
  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()
output: 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()