Use from __future__ import annotations in generate_stubs.py

regex.Match isn't actually accessible, so without this the file does not run.

Author: asmeurer

generate_stubs.py

@@ -9,6 +9,8 @@
 
 This will update the stub files in array_api_tests/function_stubs/
 """
+from __future__ import annotations
+
 import argparse
 import os
 import ast