[test] Use pytest instead of nosetests (#482)

`nosetests` is no longer being maintained : https://github.com/nose-devs/nose/issues/1099
and will stop working in py 3.10 as can be seen in #480

test/test_write_annotations.py

@@ -8,7 +8,7 @@
 import unittest
 sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
 
-from test.helper import get_params, try_rm
+from test.helper import get_params, try_rm, is_download_test
 
 
 import io
@@ -38,6 +38,7 @@ def __init__(self, *args, **kwargs):
 EXPECTED_ANNOTATIONS = ['Speech bubble', 'Note', 'Title', 'Spotlight', 'Label']
 
 
+@is_download_test
 class TestAnnotations(unittest.TestCase):
     def setUp(self):
         # Clear old files