gnu: Add python2-objgraph.

* gnu/packages/python-xyz.scm (python2-objgraph): New public variable.

gnu/packages/python-xyz.scm

@@ -10006,6 +10006,9 @@ (define-public python-objgraph
 graphviz.")
     (license license:expat)))
+(define-public python2-objgraph
+  (package-with-python2 python-objgraph))
+
 (define-public python-gevent
   (package
     (name "python-gevent")