[oppetarkiv] Add new extractor

Some, if not all, of the videos appear to be geo-blocked (Sweden).
Test might fail (403 Forbidden) if not run through a Swedish connection.

youtube_dl/extractor/__init__.py

@@ -350,6 +350,7 @@ from .odnoklassniki import OdnoklassnikiIE
 from .oktoberfesttv import OktoberfestTVIE
 from .ooyala import OoyalaIE
 from .openfilm import OpenFilmIE
+from .oppetarkiv import OppetArkivIE
 from .orf import (
     ORFTVthekIE,
     ORFOE1IE,