fix: py38/39 typing compatibility

constraints-oldest.txt

@@ -20,4 +20,5 @@ ruamel.yaml==0.17.21
 toml==0.10.0
 twine==2.0.0
 types-toml==0.10.4
+typing_extensions==4.0.1
 wheel==0.21.0

setup.cfg

@@ -31,6 +31,7 @@ install_requires =
     black>=22.3.0
     darkgraylib~=2.0.1
     toml>=0.10.0
+    typing_extensions>=4.0.1
 # NOTE: remember to keep `.github/workflows/python-package.yml` in sync
 #       with the minimum required Python version
 python_requires = >=3.8

src/darker/config.py

@@ -2,16 +2,22 @@
 
 from __future__ import annotations
 
+import sys
 from dataclasses import dataclass, field
 from pathlib import Path
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Union
 
 from darkgraylib.config import BaseConfig, ConfigurationError
 
+if sys.version_info < (3, 10):
+    from typing_extensions import TypeAlias
+else:
+    from typing import TypeAlias
+
 if TYPE_CHECKING:
     from argparse import Namespace
 
-UnvalidatedConfig = dict[str, list[str] | str | bool | int]
+UnvalidatedConfig: TypeAlias = dict[str, Union[list[str], str, bool, int]]
 
 
 REMOVED_CONFIG_OPTIONS = {