tools.memories, test: fix del SopelIdentifierMemory['key'] behavior

Resolves `KeyError` currently raised if the `key` parameter (`str`)
doesn't match the internal (lowercased) representation of `Identifier`.

That is, `del an_identifier_memory['key']` might have worked, but doing
`del an_identifier_memory['KeY']` would not. This is now fixed.

Author: dgw

sopel/tools/memories.py

@@ -182,6 +182,9 @@ def __contains__(self, key):
     def __setitem__(self, key: Optional[str], value):
         super().__setitem__(self._make_key(key), value)
 
+    def __delitem__(self, key: str):
+        super().__delitem__(self._make_key(key))
+
     def pop(self, key: str, default=_NO_DEFAULT):
         if default is _NO_DEFAULT:
             return super().pop(self._make_key(key))

test/tools/test_tools_memories.py

@@ -123,3 +123,11 @@ def test_sopel_identifier_memory_pop():
     assert 'missing_key' not in pop_me
     popped = pop_me.pop('missing_key', 'DEFAULT')
     assert popped == 'DEFAULT'
+
+
+def test_sopel_identifier_memory_del_key():
+    memory = memories.SopelIdentifierMemory()
+    memory['KeY'] = True
+    assert 'KeY' in memory
+    del memory['KeY']
+    assert 'KeY' not in memory