Remove deprecated behavior from dataset.drop docstring (#3451)

* remove deprecated behavior from dataset.drop docstring

* remove a few warnings too

* actually keep original form but test for warnings

xarray/core/dataset.py

@@ -3542,7 +3542,6 @@ def drop(  # noqa: F811
         ----------
         labels : hashable or iterable of hashables
             Name(s) of variables or index labels to drop.
-            If dim is not None, labels can be any array-like.
         dim : None or hashable, optional
             Dimension along which to drop index labels. By default (if
             ``dim is None``), drops variables rather than index labels.

xarray/tests/test_dataset.py

@@ -2117,25 +2117,31 @@ def test_drop_variables(self):
     def test_drop_index_labels(self):
         data = Dataset({"A": (["x", "y"], np.random.randn(2, 3)), "x": ["a", "b"]})
 
-        actual = data.drop(["a"], "x")
+        with pytest.warns(DeprecationWarning):
+            actual = data.drop(["a"], "x")
         expected = data.isel(x=[1])
         assert_identical(expected, actual)
 
-        actual = data.drop(["a", "b"], "x")
+        with pytest.warns(DeprecationWarning):
+            actual = data.drop(["a", "b"], "x")
         expected = data.isel(x=slice(0, 0))
         assert_identical(expected, actual)
 
         with pytest.raises(KeyError):
             # not contained in axis
-            data.drop(["c"], dim="x")
+            with pytest.warns(DeprecationWarning):
+                data.drop(["c"], dim="x")
 
-        actual = data.drop(["c"], dim="x", errors="ignore")
+        with pytest.warns(DeprecationWarning):
+            actual = data.drop(["c"], dim="x", errors="ignore")
         assert_identical(data, actual)
 
         with pytest.raises(ValueError):
-            data.drop(["c"], dim="x", errors="wrong_value")
+            with pytest.warns(DeprecationWarning):
+                data.drop(["c"], dim="x", errors="wrong_value")
 
-        actual = data.drop(["a", "b", "c"], "x", errors="ignore")
+        with pytest.warns(DeprecationWarning):
+            actual = data.drop(["a", "b", "c"], "x", errors="ignore")
         expected = data.isel(x=slice(0, 0))
         assert_identical(expected, actual)