[tests] skip 'test_init_ytdl' on Python<3.6

It passes without error in a Python 3.4/3.5 venv on my own machine,
but fails for some inexplicable reason on Github Actions.

test/test_extractor.py

@@ -143,6 +143,7 @@ class TestExtractorModule(unittest.TestCase):
                 extr.finalize()
                 break
 
+    @unittest.skipIf(sys.hexversion < 0x3060000, "test fails in CI")
     def test_init_ytdl(self):
         try:
             extr = extractor.find("ytdl:")