add type hints for _util

pyproject.toml

@@ -37,6 +37,9 @@ classifiers = [
 dynamic = [
   "version",
 ]
+dependencies = [
+  'typing-extensions; python_version < "3.10"',
+]
 [project.optional-dependencies]
 docs = [
   "furo",

src/PIL/_util.py

@@ -1,21 +1,32 @@
 from __future__ import annotations
 
 import os
+import sys
 from pathlib import Path
+from typing import Any, NoReturn
 
+if sys.version_info >= (3, 10):
+    from typing import TypeGuard
+else:
+    from typing_extensions import TypeGuard
 
-def is_path(f):
+
+def is_path(f: Any) -> TypeGuard[bytes | str | Path]:
     return isinstance(f, (bytes, str, Path))
 
 
-def is_directory(f):
+def is_directory(f: Any) -> TypeGuard[bytes | str | Path]:
     """Checks if an object is a string, and that it points to a directory."""
     return is_path(f) and os.path.isdir(f)
 
 
 class DeferredError:
-    def __init__(self, ex):
+    def __init__(self, ex: BaseException):
         self.ex = ex
 
-    def __getattr__(self, elt):
+    def __getattr__(self, elt: str) -> NoReturn:
         raise self.ex
+
+    @staticmethod
+    def new(ex: BaseException) -> Any:
+        return DeferredError(ex)