docs: def encode_bu64(b):
      """Encode bytes to a URL safe flavor of Base64 used by JWTs.
  
      - Reverse of decode_bu64().
  
      Args:
        b: bytes
          Bytes to Base64 encode.
  
      Returns:
        bytes: URL safe Base64 encoded version of input.
  
      """
      s = base64.standard_b64encode(b)
      s = s.rstrip('=')
      s = s.replace('+', '-')
      s = s.replace('/', '_')
      return s