use documented hmac.new API

rather than class constructor, which is not compatible with both Python 2 and 3

Author: minrk

notebook/notebookapp.py

@@ -706,7 +706,7 @@ def _default_cookie_secret(self):
         else:
             key = encodebytes(os.urandom(32))
             self._write_cookie_secret_file(key)
-        h = hmac.HMAC(key, digestmod='sha256')
+        h = hmac.new(key, digestmod='sha256')
         h.update(self.password.encode())
         return h.digest()