[amcnetworks] Fix extractor (#179)

* Prefer use of manifest based on `releasePid` since the one based on `videoPid` may have Fairplay
* Additional thumbnail images were added
* Don't add `season_number` and `series` to `title`
* `series` is now set to `None` rather than "_" when empty
* fix bug with age limit

Authored by: 2ShedsJackson

yt_dlp/utils.py

@@ -4106,6 +4106,7 @@ def parse_age_limit(s):
     m = re.match(r'^(?P<age>\d{1,2})\+?$', s)
     if m:
         return int(m.group('age'))
+    s = s.upper()
     if s in US_RATINGS:
         return US_RATINGS[s]
     m = re.match(r'^TV[_-]?(%s)$' % '|'.join(k[3:] for k in TV_PARENTAL_GUIDELINES), s)