Python: Add nullary pointsTo to Expr class

Like the one existing in ControlFlowNode.

This is useful for checking class of value being poitned to, as

    expr.pointsTo().getClass() = someClass

Without this you need to do

    exists(Value v | v.getClass() = someClass | expr.pointsTo(v))

python/ql/src/semmle/python/Exprs.qll

@@ -126,6 +126,11 @@ class Expr extends Expr_, AstNode {
         this.pointsTo(value, _)
     }
 
+    /** Gets a value that this expression might "point-to". */
+    Value pointsTo() {
+        this.pointsTo(result)
+    }
+
 }
 
 /** An attribute expression, such as `value.attr` */