fixes documentation generation

.readthedocs.yml

@@ -15,3 +15,5 @@ formats: all
 # Optionally set the version of Python and requirements required to build your docs
 python:
   version: 3.7
+  install:
+    - requirements: docs/requirements.txt

docs/requirements.txt

@@ -0,0 +1 @@
+typing_extensions

pytube/monostate.py

@@ -1,10 +1,6 @@
 import io
 from typing import Any, Optional
-
-try:
-    from typing import Protocol
-except ImportError:
-    from typing_extensions import Protocol
+from typing_extensions import Protocol
 
 # from __future__ import annotations