fix: Restore self in class method

Author: alecandido

src/qibo/transpiler/unroller.py

@@ -20,10 +20,10 @@
 class FlagMeta(EnumMeta):
     """Metaclass for :class:`qibo.transpiler.unroller.NativeGates` that allows initialization with a list of gate name strings."""
 
-    def __getitem__(cls, keys):
+    def __getitem__(self, keys):
         if isinstance(keys, str):
             return super().__getitem__(keys)
-        return reduce(or_, [cls[key] for key in keys])  # pylint: disable=E1136
+        return reduce(or_, [self[key] for key in keys])  # pylint: disable=E1136
 
 
 class NativeGates(Flag, metaclass=FlagMeta):