[cleanup] Misc (#11216)

- Add Python 3.13 to CI, finalize 3.13 support
- Remove Python 3.8 from CI in preparation for removing 3.8 support
- Document that PyPy3.8 and PyPy3.9 are no longer supported
- Usual documentation fixes and code cleanup

Closes #8248, Closes #11146, Closes #11149, Closes #11211
Authored by: Grub4K, grqz, DTrombett, KarboniteKream, bashonly, mikkovedru, seproDev

Co-authored-by: N/Ame <173015200+grqz@users.noreply.github.com>
Co-authored-by: DTrombett <d@trombett.org>
Co-authored-by: =?UTF-8?q?Klemen=20Ko=C5=A1ir?= <klemen.kosir@kream.io>
Co-authored-by: bashonly <88596187+bashonly@users.noreply.github.com>
Co-authored-by: Mikko Vedru <mikko.vedru@gmail.com>
Co-authored-by: sepro <sepro@sepr0.com>

yt_dlp/extractor/tubetugraz.py

@@ -236,7 +236,7 @@ class TubeTuGrazSeriesIE(TubeTuGrazBaseIE):
                 },
             },
         ],
-        'min_playlist_count': 4,
+        'playlist_mincount': 4,
     }]
 
     def _real_extract(self, url):