Add how_old.py example for chapter 3

PY4E/ch03/how_old.py

@@ -0,0 +1,8 @@
+age = input("How old are you? ")
+
+while not age.isnumeric():
+    age = input("Please enter only digits. Try again. How old are you? ")
+
+age = int(age)
+print("Great, your are", age, "years old. Next year you'll be", age+1, "!")
+