input: Write a python function to encode a string to base64.
  assert encode_base64("hello") == b'aGVsbG8='
  assert encode_base64("world") == b'd29ybGQ='
  assert encode_base64("python") == b'cHl0aG9u'
output: def encode_base64(s: str) -> bytes:
      import base64
      return base64.b64encode(s.encode())