[NRKTV] Added season and episode number to metadata.
[NRKTV] Added category to metadata.
[NRKTV] Added tests to NRKTVSeries.
[NRKTV] Fixed whitespace issues (flake8).
HTMLParser, which is used by extract_attributes, already unescapes
attribute values with HTMLParser.unescape. They shouldn't be unescaped
again, to there may be parsing errors.
Ref: #11219, #11522