Add missing override annotations.

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

@@ -502,7 +502,7 @@ class Dict extends Dict_ {
         result = this.getAValue() or result = this.getAKey()
     }
 
-    AstNode getAChildNode() {
+    override AstNode getAChildNode() {
         result = this.getAnItem()
     }
 

python/ql/src/semmle/python/types/ClassObject.qll

@@ -385,7 +385,7 @@ class ClassObject extends Object {
         result.getFunction().refersTo(this)
     }
 
-    predicate notClass() {
+    override predicate notClass() {
         none()
     }