code: def request_signature(self, stringtosign):
          
          digest = hmac.new(
              self.secret_key.encode(ENCODING),
              stringtosign.encode(ENCODING),
              hashlib.sha1
          ).digest()
          return b64_string(digest)
docs: Construct a signature by making an RFC2104 HMAC-SHA1
          of the following and converting it to Base64 UTF-8 encoded string.