input: Write a python function to generate a JWT token for a given username and secret key.
  assert generate_token('user1', 'mysecret') == jwt.encode({'username': 'user1'}, 'mysecret')
  assert generate_token('admin', 'admin_secret') == jwt.encode({'username': 'admin'}, 'admin_secret')
  assert generate_token('guest', 'guest_key') == jwt.encode({'username': 'guest'}, 'guest_key')
output: import jwt
  
  def generate_token(username, secret_key):
      payload = {'username': username}
      token = jwt.encode(payload, secret_key)
      return token