TYP: use typing.final in indexes.base (#37527)

Author: jbrockmendel

pandas/_typing.py

@@ -27,6 +27,8 @@
 # and use a string literal forward reference to it in subsequent types
 # https://mypy.readthedocs.io/en/latest/common_issues.html#import-cycles
 if TYPE_CHECKING:
+    from typing import final
+
     from pandas._libs import Period, Timedelta, Timestamp
 
     from pandas.core.dtypes.dtypes import ExtensionDtype
@@ -42,6 +44,10 @@
     from pandas.core.window.rolling import BaseWindow
 
     from pandas.io.formats.format import EngFormatter
+else:
+    # typing.final does not exist until py38
+    final = lambda x: x
+
 
 # array-like