[CI] Ensure compatibility with Python 3.8

55436ae broke this on Windows as we
only use Python 3.9 there, but the construct is only supported from 3.10
onwards. Use the old Optional type to ensure compatibility.

Author: boomanaiden154

.ci/generate_test_report_lib.py

@@ -3,18 +3,20 @@
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 """Library to parse JUnit XML files and return a markdown report."""
 
-from typing import TypedDict
+from typing import TypedDict, Optional
 import platform
 
 from junitparser import JUnitXml, Failure
 
 
 # This data structure should match the definition in llvm-zorg in
 # premerge/advisor/advisor_lib.py
+# TODO(boomanaiden154): Drop the Optional here and switch to str | None when
+# we require Python 3.10.
 class FailureExplanation(TypedDict):
     name: str
     explained: bool
-    reason: str | None
+    reason: Optional[str]
 
 
 SEE_BUILD_FILE_STR = "Download the build's log file to see the details."