input: Write a function to create a web API with Python and Flask to provide the current date and time. Define the function with the name create_datetime_api and have it return the Flask app instance. The API should respond at the route '/datetime' with the current date and time formatted as "YYYY-MM-DD HH:MM:SS".
  
  from flask import Flask
  import datetime
  
  def create_datetime_api():
      app = Flask(__name__)
      
      @app.route('/datetime')
      def get_datetime():
          return datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S")
      
      return app
  
  assert create_datetime_api().test_client().get('/datetime').status_code == 200
  assert '1970' in create_datetime_api().test_client().get('/datetime').data.decode()
output: 
  def create_datetime_api():
      app = Flask(__name__)
      
      @app.route('/datetime')
      def get_datetime():
          return datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S")
      
      return app
  