Fix type error in validate_sphinx script (#2665)

Summary:
Pull Request resolved: #2665

Reverts part of D61274223

Reviewed By: mpolson64

Differential Revision: D61388107

fbshipit-source-id: 7e98aa0ca82aa3e21a5d57ffa6ef0a43a4beeecd

scripts/validate_sphinx.py

@@ -8,6 +8,7 @@
 import os
 import pkgutil
 import re
+from typing import Set
 
 
 # Paths are relative to top-level Ax directory (which is passed into fxn below)
@@ -28,7 +29,7 @@
 }
 
 
-def parse_rst(rst_filename: str) -> set[str]:
+def parse_rst(rst_filename: str) -> Set[str]:
     """Extract automodule directives from rst."""
     ret = set()
     with open(rst_filename, "r") as f: