input: Write a python function to retrieve a user by username from the database.
  assert get_user_by_username("testuser").username == "testuser"
  assert get_user_by_username("nonexistentuser") is None
  assert get_user_by_username("anotheruser").email == "anotheruser@example.com"
output: def get_user_by_username(username):
      import sqlite3
  
      # Assume the database is named 'users.db' and the table is named 'users'
      conn = sqlite3.connect('users.db')
      cursor = conn.cursor()
      
      cursor.execute("SELECT username, email FROM users WHERE username = ?", (username,))
      row = cursor.fetchone()
      
      conn.close()
      
      if row:
          class User:
              def __init__(self, username, email):
                  self.username = username
                  self.email = email
                  
          return User(row[0], row[1])
      else:
          return None