bpo-31029: test_tokenize Add missing import unittest (#2998)

Author: rajathagasthya

Lib/test/test_tokenize.py

@@ -3,6 +3,7 @@
                      STRING, ENDMARKER, ENCODING, tok_name, detect_encoding,
                      open as tokenize_open, Untokenizer)
 from io import BytesIO
+import unittest
 from unittest import TestCase, mock
 from test.test_grammar import (VALID_UNDERSCORE_LITERALS,
                                INVALID_UNDERSCORE_LITERALS)