Fix exception for del config[key] (#7391)

* Fix exception for `del config[key]`

Also, make `cfg.__setitem__` faster

* Fix regex in test for Python 3.10

networkx/utils/configs.py

@@ -128,11 +128,16 @@ def __getitem__(self, key):
 
     def __setitem__(self, key, value):
         try:
-            setattr(self, key, value)
+            self.__setattr__(key, value)
+        except AttributeError as err:
+            raise KeyError(*err.args) from None
+
+    def __delitem__(self, key):
+        try:
+            self.__delattr__(key)
         except AttributeError as err:
             raise KeyError(*err.args) from None
 
-    __delitem__ = __delattr__
     _ipython_key_completions_ = __dir__  # config["<TAB>
 
     # Go ahead and make it a `collections.abc.Mapping`

networkx/utils/tests/test_config.py

@@ -169,6 +169,10 @@ class FlexibleConfig(Config, strict=False):
     del cfg["y"]
     assert len(cfg) == 0
     assert list(cfg) == []
+    with pytest.raises(AttributeError, match="y"):
+        del cfg.y
+    with pytest.raises(KeyError, match="y"):
+        del cfg["y"]
     with pytest.raises(TypeError, match="missing 1 required keyword-only"):
         FlexibleConfig()
     # Be strict when first creating the config object