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". Implement the function to define the API endpoint '/datetime' which when accessed will return the current date and time as a string.
  assert get_datetime()=="2023-10-05 14:30:00"
  assert get_datetime()=="2023-10-05 14:31:00"
  assert get_datetime()=="2023-10-05 14:32:00"
  assert get_datetime()=="2023-10-05 14:33:00"
  assert get_datetime()=="2023-10-05 14:34:00"
output: 
  from flask import Flask
  from datetime import datetime
  
  app = Flask(__name__)
  
  @app.route('/datetime')
  def get_datetime():
      return datetime.now().strftime("%Y-%m-%d %H:%M:%S")
  