[tests] a HTTP 503 is a transient issue

test/test_download.py

@@ -26,6 +26,7 @@ import youtube_dl.YoutubeDL
 from youtube_dl.utils import (
     compat_str,
     compat_urllib_error,
+    compat_HTTPError,
     DownloadError,
     ExtractorError,
     UnavailableVideoError,
@@ -105,7 +106,7 @@ def generator(test_case):
                     ydl.download([test_case['url']])
                 except (DownloadError, ExtractorError) as err:
                     # Check if the exception is not a network related one
-                    if not err.exc_info[0] in (compat_urllib_error.URLError, socket.timeout, UnavailableVideoError):
+                    if not err.exc_info[0] in (compat_urllib_error.URLError, socket.timeout, UnavailableVideoError) or (err.exc_info[0] == compat_HTTPError and err.exc_info[1].code == 503):
                         raise
 
                     if try_num == RETRIES: