[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_post_hooks.py

@@ -7,7 +7,7 @@
 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 yt_dlp.YoutubeDL
 from yt_dlp.utils import DownloadError
 
@@ -22,6 +22,7 @@ def __init__(self, *args, **kwargs):
 EXPECTED_NAME = 'gr51aVj-mLg'
 
 
+@is_download_test
 class TestPostHooks(unittest.TestCase):
     def setUp(self):
         self.stored_name_1 = None