CI: Fix tests broken by np 1.18 sorting change (pandas-dev#29877)

Author: jbrockmendel

pandas/core/indexes/datetimelike.py

@@ -284,7 +284,10 @@ def sort_values(self, return_indexer=False, ascending=True):
             sorted_index = self.take(_as)
             return sorted_index, _as
         else:
-            sorted_values = np.sort(self._ndarray_values)
+            # NB: using asi8 instead of _ndarray_values matters in numpy 1.18
+            #  because the treatment of NaT has been changed to put NaT last
+            #  instead of first.
+            sorted_values = np.sort(self.asi8)
             attribs = self._get_attributes_dict()
             freq = attribs["freq"]