[NRKTV] Added NRKTVSeriesIE

[NRKTV] Added season and episode number to metadata.

[NRKTV] Added category to metadata.

[NRKTV] Added tests to NRKTVSeries.

[NRKTV] Fixed whitespace issues (flake8).

youtube_dl/extractor/extractors.py

@@ -668,6 +668,7 @@ from .nrk import (
     NRKTVIE,
     NRKTVDirekteIE,
     NRKTVEpisodesIE,
+    NRKTVSeriesIE,
 )
 from .ntvde import NTVDeIE
 from .ntvru import NTVRuIE