[libc++] Fixes building with Python 3.8.

This addresses the post-commit issues reported in #101880.

Author: mordante

libcxx/utils/generate_feature_test_macro_components.py

@@ -1957,7 +1957,7 @@ def produce_docs():
 
 @dataclass
 class Metadata:
-    headers: list[str] = None
+    headers: List[str] = None
     test_suite_guard: str = None
     libcxx_guard: str = None