input: Write a python function to encode a given string to base64.
  assert base64_encode("hello") == "aGVsbG8="
  assert base64_encode("world") == "d29ybGQ="
  assert base64_encode("python") == "cHl0aG9u"
output: import base64
  
  def base64_encode(s):
      return base64.b64encode(s.encode()).decode()