[sys] Deprecate _enablelegacywindowsfsencoding (#14495)

Author: donbarbos

stdlib/sys/__init__.pyi

@@ -454,7 +454,14 @@ def get_asyncgen_hooks() -> _asyncgen_hooks: ...
 def set_asyncgen_hooks(firstiter: _AsyncgenHook = ..., finalizer: _AsyncgenHook = ...) -> None: ...
 
 if sys.platform == "win32":
-    def _enablelegacywindowsfsencoding() -> None: ...
+    if sys.version_info >= (3, 13):
+        @deprecated(
+            "Deprecated since Python 3.13; will be removed in Python 3.16. "
+            "Use the `PYTHONLEGACYWINDOWSFSENCODING` environment variable instead."
+        )
+        def _enablelegacywindowsfsencoding() -> None: ...
+    else:
+        def _enablelegacywindowsfsencoding() -> None: ...
 
 def get_coroutine_origin_tracking_depth() -> int: ...
 def set_coroutine_origin_tracking_depth(depth: int) -> None: ...