[test_youtube_signature] Fix import

Broken in commit 8c25f81beea169c9d6540eea1a6f71dc045da6ed

test/test_youtube_signature.py

@@ -14,7 +14,7 @@ import re
 import string
 
 from youtube_dl.extractor import YoutubeIE
-from youtube_dl.utils import compat_str, compat_urlretrieve
+from youtube_dl.compat import compat_str, compat_urlretrieve
 
 _TESTS = [
     (