fix tests - flagged as slow all the tests downloading from AWS

transformers/tests/tokenization_distilbert_test.py

@@ -16,6 +16,7 @@ from __future__ import absolute_import, division, print_function, unicode_litera
 
 import os
 import unittest
+import pytest
 from io import open
 
 from transformers.tokenization_distilbert import (DistilBertTokenizer)
@@ -30,6 +31,7 @@ class DistilBertTokenizationTest(BertTokenizationTest):
     def get_tokenizer(self, **kwargs):
         return DistilBertTokenizer.from_pretrained(self.tmpdirname, **kwargs)
 
+    @pytest.mark.slow
     def test_sequence_builders(self):
         tokenizer = DistilBertTokenizer.from_pretrained("distilbert-base-uncased")