🩹 Improve type guard for +=

Co-authored-by: ddelange <[email protected]>

src/retrie/trie.py

@@ -87,7 +87,7 @@ def __iadd__(
         other,  # type: "Trie"
     ):  # type: (...) -> "Trie"
         """Merge another Trie object into the current Trie."""
-        if not isinstance(other, Trie):
+        if self.__class__ != other.__class__:
             raise TypeError(
                 "Unsupported operand type(s) for +=: '{0}' and '{1}'".format(
                     type(self), type(other)