Python: Add getObject(string attrName) to AttrRef

Now that I got started adding small things that are nice, I've been
missing this one (that is available on an `AttrNode`).

Author: RasmusWL

python/ql/src/semmle/python/dataflow/new/internal/Attributes.qll

@@ -17,6 +17,15 @@ abstract class AttrRef extends Node {
    */
   abstract Node getObject();
 
+  /**
+   * Gets the data flow node corresponding to the object whose attribute named
+   * `attrName` is being read or written.
+   */
+  Node getObject(string attrName) {
+    result = this.getObject() and
+    attrName = this.getAttributeName()
+  }
+
   /**
    * Gets the expression node that defines the attribute being accessed, if any. This is
    * usually an identifier or literal.