adapt to NEP 51 (pydata#8064)

* convert string and bytes items to standard python types

* [test-upstream]

* modify the expected error message

xarray/core/formatting.py

@@ -160,6 +160,8 @@ def format_item(x, timedelta_format=None, quote_strings=True):
     if isinstance(x, (np.timedelta64, timedelta)):
         return format_timedelta(x, timedelta_format=timedelta_format)
     elif isinstance(x, (str, bytes)):
+        if hasattr(x, "dtype"):
+            x = x.item()
         return repr(x) if quote_strings else x
     elif hasattr(x, "dtype") and np.issubdtype(x.dtype, np.floating):
         return f"{x.item():.4}"

xarray/tests/test_dataset.py

@@ -4116,7 +4116,8 @@ def test_setitem(self) -> None:
             data4[{"dim2": [2, 3]}] = data3["var1"][{"dim2": [3, 4]}].values
         data5 = data4.astype(str)
         data5["var4"] = data4["var1"]
-        err_msg = "could not convert string to float: 'a'"
+        # convert to `np.str_('a')` once `numpy<2.0` has been dropped
+        err_msg = "could not convert string to float: .*'a'.*"
         with pytest.raises(ValueError, match=err_msg):
             data5[{"dim2": 1}] = "a"