youtube-dl: Update to 20210417

Changes:
20210417
--------
Core
+ [utils] Add support for experimental HTTP response status code
  308 Permanent Redirect (#27877, #28768)

Extractors
+ [lbry] Add support for HLS videos (#27877, #28768)
* [youtube] Fix stretched ratio calculation
* [youtube] Improve stretch extraction (#28769)
* [youtube:tab] Improve grid extraction (#28725)
+ [youtube:tab] Detect series playlist on playlists page (#28723)
+ [youtube] Add more invidious instances (#28706)
* [pluralsight] Extend anti-throttling timeout (#28712)
* [youtube] Improve URL to extractor routing (#27572, #28335, #28742)
+ [maoritv] Add support for maoritelevision.com (#24552)
+ [youtube:tab] Pass innertube context and x-goog-visitor-id header along with
  continuation requests (#28702)
* [mtv] Fix Viacom A/B Testing Video Player extraction (#28703)
+ [pornhub] Extract DASH and HLS formats from get_media end point (#28698)
* [cbssports] Fix extraction (#28682)
* [jamendo] Fix track extraction (#28686)
* [curiositystream] Fix format extraction (#26845, #28668)

(leot)