input: Write a python function to insert a new user into the 'users' table in the 'user_info.db' database.
  assert insert_user('user_info.db', 'Alice', 25, 'F', 'a1l2i3c4e') is None
  assert insert_user('user_info.db', 'Bob', 30, 'M', 'b1o2b3') is None
output: def insert_user(db_name, name, age, gender, username):
      import sqlite3
  
      # Connect to the database
      conn = sqlite3.connect(db_name)
      cursor = conn.cursor()
  
      # Create the users table if it doesn't exist
      cursor.execute('''
      CREATE TABLE IF NOT EXISTS users (
          id INTEGER PRIMARY KEY AUTOINCREMENT,
          name TEXT NOT NULL,
          age INTEGER NOT NULL,
          gender TEXT NOT NULL,
          username TEXT NOT NULL UNIQUE
      )
      ''')
  
      # Insert the new user into the users table
      cursor.execute('''
      INSERT INTO users (name, age, gender, username)
      VALUES (?, ?, ?, ?)
      ''', (name, age, gender, username))
  
      # Commit the changes and close the connection
      conn.commit()
      conn.close()