fix: relax type constaint in mutability check

Author: AayushSabharwal

src/interface.jl

@@ -261,7 +261,7 @@ Return either [`IsMutable`](@ref) to indicate an object of type `T` can be
 modified to be equal to `op(args...)` or [`IsNotMutable`](@ref) otherwise.
 """
 function mutability(T::Type, op, args::Vararg{Type,N}) where {N}
-    if mutability(T) isa IsMutable && promote_operation(op, args...) == T
+    if mutability(T) isa IsMutable && promote_operation(op, args...) <: T
         return IsMutable()
     else
         return IsNotMutable()