Add workaround for old python not supporting ISO 8601 datetimes

This should be removed when we drop python 3.10 support.

src/CSET/_workflow_utils/fetch_data.py

@@ -7,6 +7,7 @@
 import logging
 import os
 import shutil
+import sys
 from concurrent.futures import ThreadPoolExecutor
 from datetime import datetime, timedelta
 from typing import Literal
@@ -85,10 +86,16 @@ def get_file(self, file_path: str, output_dir: str) -> None:
 
 def _get_needed_environment_variables() -> dict:
     """Load the needed variables from the environment."""
+    # Python 3.10 and older don't fully support ISO 8601 datetime formats.
+    # TODO: Remove once we drop python 3.10.
+    if sys.version_info.minor < 11:
+        _fromisoformat = isodate.parse_datetime
+    else:
+        _fromisoformat = datetime.fromisoformat
     variables = {
         "raw_path": os.environ["DATA_PATH"],
         "date_type": os.environ["DATE_TYPE"],
-        "data_time": datetime.fromisoformat(os.environ["CYLC_TASK_CYCLE_POINT"]),
+        "data_time": _fromisoformat(os.environ["CYLC_TASK_CYCLE_POINT"]),
         "forecast_length": isodate.parse_duration(os.environ["CSET_ANALYSIS_PERIOD"]),
         "forecast_offset": isodate.parse_duration(os.environ["CSET_ANALYSIS_OFFSET"]),
         "share_dir": os.environ["CYLC_WORKFLOW_SHARE_DIR"],