Raise correct exception out of Spines.__getattr__

If the name does not exist, then `__getattr__` should raise
`AttributeError`, not `ValueError`.

Fixes matplotlib#21554

Author: QuLogic

lib/matplotlib/spines.py

@@ -550,7 +550,7 @@ def __getattr__(self, name):
         try:
             return self._dict[name]
         except KeyError:
-            raise ValueError(
+            raise AttributeError(
                 f"'Spines' object does not contain a '{name}' spine")
 
     def __getitem__(self, key):

lib/matplotlib/tests/test_spines.py

@@ -35,6 +35,8 @@ def set_val(self, val):
     spines[:].set_val('y')
     assert all(spine.val == 'y' for spine in spines.values())
 
+    with pytest.raises(AttributeError, match='foo'):
+        spines.foo
     with pytest.raises(KeyError, match='foo'):
         spines['foo']
     with pytest.raises(KeyError, match='foo, bar'):