adding iteration code examples

Chapter6/iter_util/itertools_groupby.py

@@ -0,0 +1,9 @@
+#itertools_groupby.py
+import itertools
+
+mylist = [("A", 100), ("A", 200), ("B", 30), ("B", 10)]
+def get_key(group):
+    return group[0]
+
+for key, grp in itertools.groupby(mylist, get_key):
+    print(key + "-->", list(grp))