New regression: duplicate IE name npo

[anisse]

Since commit a3d8b38 (ping @dstftw ), test_no_duplicated_ie_names is now failing:

Traceback (most recent call last):
  File "/home/aissen/dev/youtube-dl-dev-new/test/test_all_urls.py", line 139, in test_no_duplicated_ie_names
    'Multiple extractors with the same IE_NAME "%s" (%s)' % (ie_name, ', '.join(ie_list)))
AssertionError: 3 != 1 : Multiple extractors with the same IE_NAME "npo" (AndereTijdenIE, WNLIE, NPOIE)

See this build:
https://travis-ci.org/anisse/youtube-dl/jobs/162634751