Switch Flask library from flask_restful to flask_restx in app.py

Flask_restful library is replaced with flask_restx in app.py file to leverage extended functionalities provided by flask_restx. This change aims to refine the resource initialization and streamline the handling of database connections.

app.py

@@ -1,5 +1,5 @@
 from flask import Flask
-from flask_restful import Api
+from flask_restx import Api
 from flask_cors import CORS
 from resources.User import User
 from resources.Login import Login