add explicit arg.deleter raising AttributeError for cdef class compat

In Cython cdef class, a property without a deleter raises
NotImplementedError instead of Python's AttributeError. Add an explicit
deleter to BaseInstr.arg that raises AttributeError to keep consistent
behaviour across pure Python and Cython builds.

Author: P403n1x87

src/bytecode/instr.py

@@ -788,6 +788,10 @@ def arg(self) -> A:
     def arg(self, arg: A):
         self._set(self._name, arg)
 
+    @arg.deleter
+    def arg(self) -> None:
+        raise AttributeError("can't delete attribute")
+
     @property
     def lineno(self) -> int | _UNSET | None:
         return self._location.lineno if self._location is not None else UNSET