Change np.core.defchararray to np.char.chararray (pydata#9165)

Replace a reference to np.core.defchararray with np.char.chararray
in xarray.testing.assertions, since the former no longer works on
NumPy 2.0.0 and the latter is the "preferred alias" according to
NumPy docs. See Issue pydata#9165.

xarray/testing/assertions.py

@@ -36,7 +36,7 @@ def wrapper(*args, **kwargs):
 
 def _decode_string_data(data):
     if data.dtype.kind == "S":
-        return np.core.defchararray.decode(data, "utf-8", "replace")
+        return np.char.chararray.decode(data, "utf-8", "replace")
     return data