Work around StrEnum not being in python until 3.11

pyproject.toml

@@ -9,7 +9,7 @@ readme = "README.md"
 license = { text = "Apache-2" }
 requires-python = ">= 3.9"
 dynamic = ["version"]
-dependencies = ["lark[interegular]", "numpy", "convertdate"]
+dependencies = ["lark[interegular]", "numpy", "convertdate", "strenum; python_version > '3.11'"]
 authors = [
     { name = "Rebecca Sutton Koeser" },
     { name = "Cole Crawford" },

src/undate/undate.py

@@ -1,6 +1,14 @@
 import datetime
 import re
-from enum import StrEnum, auto
+
+from enum import auto
+
+try:
+    # StrEnum was only added in python 3.11
+    from enum import StrEnum
+except ImportError:
+    # for python 3.10 or earlier, use third-party package
+    from strenum import StrEnum  # type: ignore
 
 # Pre 3.10 requires Union for multiple types, e.g. Union[int, None] instead of int | None
 from typing import Dict, Optional, Union