[TeacherTubeIE] Add extractor for teachertube.com videos + classrooms (fixes #3046)

youtube_dl/extractor/__init__.py

@@ -266,6 +266,10 @@ from .streamcz import StreamCZIE
 from .swrmediathek import SWRMediathekIE
 from .syfy import SyfyIE
 from .sztvhu import SztvHuIE
+from .teachertube import (
+    TeacherTubeIE,
+    TeacherTubeClassroomIE,
+)
 from .teamcoco import TeamcocoIE
 from .techtalks import TechTalksIE
 from .ted import TEDIE