CI: troubleshoot failures that necessited #27536 (#27545)

Author: jbrockmendel

pandas/_libs/index.pyx

@@ -545,7 +545,7 @@ cpdef convert_scalar(ndarray arr, object value):
     elif arr.descr.type_num == NPY_TIMEDELTA:
         if util.is_array(value):
             pass
-        elif isinstance(value, timedelta):
+        elif isinstance(value, timedelta) or util.is_timedelta64_object(value):
             return Timedelta(value).value
         elif util.is_datetime64_object(value):
             # exclude np.datetime64("NaT") which would otherwise be picked up

pandas/tests/series/indexing/test_indexing.py

@@ -690,13 +690,7 @@ def test_dt64_series_assign_nat(nat_val, should_cast, tz):
     "nat_val,should_cast",
     [
         (pd.NaT, True),
-        pytest.param(
-            np.timedelta64("NaT", "ns"),
-            True,
-            marks=pytest.mark.xfail(
-                reason="Platform-specific failures, unknown cause", strict=False
-            ),
-        ),
+        (np.timedelta64("NaT", "ns"), True),
         (np.datetime64("NaT", "ns"), False),
     ],
 )