Add import guard for Final

Author: DanielNoord

astroid/const.py

@@ -1,6 +1,11 @@
 import enum
 import sys
-from typing import Final, FrozenSet
+from typing import FrozenSet
+
+if sys.version_info >= (3, 8):
+    from typing import Final
+else:
+    from typing_extensions import Final
 
 PY36 = sys.version_info[:2] == (3, 6)
 PY38 = sys.version_info[:2] == (3, 8)