Use stdlib typing.final in Python 3.8+ (#259)

The "final" decorator is part of the stdlib typing module since
Python 3.8.  Use it in new enough Python versions, and fall back
to typing_extensions in older versions.

Author: mgorny

CHANGES/259.misc

@@ -0,0 +1 @@
+Use stdlib typing.final in Python 3.8+, limiting typing-extensions dependency to older Python versions.

async_timeout/__init__.py

@@ -5,7 +5,11 @@
 from types import TracebackType
 from typing import Any, Optional, Type
 
-from typing_extensions import final
+
+if sys.version_info >= (3, 8):
+    from typing import final
+else:
+    from typing_extensions import final
 
 
 __version__ = "4.0.1"

setup.cfg

@@ -42,7 +42,7 @@ zip_safe = True
 include_package_data = True
 
 install_requires =
-  typing_extensions>=3.6.5
+  typing_extensions>=3.6.5; python_version < "3.8"
 
 
 [flake8]