fix: union repr setting

Signed-off-by: nstarman <nstarman@users.noreply.github.com>

Author: nstarman

plum/alias.py

@@ -128,15 +128,15 @@ def _new_str(self: object) -> str:
 def activate_union_aliases() -> None:
     """When printing `typing.Union`s, replace all aliased unions by the aliased names.
     This monkey patches `__repr__` and `__str__` for `typing.Union`."""
-    _union_type.__repr__ = _new_repr
-    _union_type.__str__ = _new_str
+    object.__setattr__(_union_type, "__repr__", _new_repr)
+    object.__setattr__(_union_type, "__str__", _new_str)
 
 
 def deactivate_union_aliases() -> None:
     """Undo what :func:`.alias.activate` did. This restores the original  `__repr__`
     and `__str__` for `typing.Union`."""
-    _union_type.__repr__ = _original_repr
-    _union_type.__str__ = _original_str
+    object.__setattr__(_union_type, "__repr__", _original_repr)
+    object.__setattr__(_union_type, "__str__", _original_str)
 
 
 _ALIASED_UNIONS: list = []