fixed GPT-2 tokenization on python 2

tests/tokenization_openai_test.py

@@ -32,7 +32,7 @@ class OpenAIGPTTokenizationTest(unittest.TestCase):
         vocab_tokens = dict(zip(vocab, range(len(vocab))))
         merges = ["#version: 0.2", "l o", "lo w", "e r</w>", ""]
         with open("/tmp/openai_tokenizer_vocab_test.json", "w") as fp:
-            json.dump(vocab_tokens, fp)
+            fp.write(json.dumps(vocab_tokens))
             vocab_file = fp.name
         with open("/tmp/openai_tokenizer_merges_test.txt", "w") as fp:
             fp.write("\n".join(merges))