input: Write a python function to encode a given string to base64.
  assert encode_base64("hello")== "aGVsbG8="
  assert encode_base64("world")== "d29ybGQ="
  assert encode_base64("Python")== "UHl0aG9u"
output: import base64
  
  def encode_base64(s: str) -> str:
      return base64.b64encode(s.encode()).decode()