input: Write a python function to encode a given string using base64.
  assert encode_base64("The quick brown fox") == b'VGhlIHF1aWNrIGJyb3duIGZveA=='
  assert encode_base64("hello world") == b'aGVsbG8gd29ybGQ='
  assert encode_base64("python") == b'cHl0aG9u'
output: import base64
  
  def encode_base64(input_string):
      return base64.b64encode(input_string.encode('utf-8'))