[extractor/BrainPOP] Add extractors (#6106)

Authored by: MinePlayersPE
Based on https://github.com/ytdl-org/youtube-dl/pull/10025

yt_dlp/extractor/_extractors.py

@@ -254,6 +254,14 @@ from .br import (
     BRMediathekIE,
 )
 from .bravotv import BravoTVIE
+from .brainpop import (
+    BrainPOPIE,
+    BrainPOPJrIE,
+    BrainPOPELLIE,
+    BrainPOPEspIE,
+    BrainPOPFrIE,
+    BrainPOPIlIE,
+)
 from .breakcom import BreakIE
 from .breitbart import BreitBartIE
 from .brightcove import (