Python: Use Builtin::special for floats ClassValue

We could find no reason for using `Builtin::builtin` instead of
`Builtin::special`. Since all the other base types use `special`, and the old
Object API is using `special`, let's also do that :)

python/ql/src/semmle/python/objects/ObjectAPI.qll

@@ -572,7 +572,7 @@ module ClassValue {
 
     /** Get the `ClassValue` for the `float` class. */
     ClassValue float_() {
-        result = TBuiltinClassObject(Builtin::builtin("float"))
+        result = TBuiltinClassObject(Builtin::special("float"))
     }
 
     /** Get the `ClassValue` for the `bytes` class (also called `str` in Python 2). */