Plugin cleanup and tweaks

plugins/youtube_download/yt_dlp/extractor/extremetube.py

@@ -1,10 +1,8 @@
-from __future__ import unicode_literals
-
 from ..utils import str_to_int
 from .keezmovies import KeezMoviesIE
 
 
-class ExtremeTubeIE(KeezMoviesIE):
+class ExtremeTubeIE(KeezMoviesIE):  # XXX: Do not subclass from concrete IE
     _VALID_URL = r'https?://(?:www\.)?extremetube\.com/(?:[^/]+/)?video/(?P<id>[^/#?&]+)'
     _TESTS = [{
         'url': 'http://www.extremetube.com/video/music-video-14-british-euro-brit-european-cumshots-swallow-652431',