Made a doctest compatible with Python 2

That test might not always execute, but can be executed when the
parent test label is explicitely given.

tests/test_discovery_sample/doctests.py

@@ -9,8 +9,8 @@ def factorial(n):
 
     >>> [factorial(n) for n in range(6)]
     [1, 1, 2, 6, 24, 120]
-    >>> factorial(30)
-    265252859812191058636308480000000
+    >>> factorial(30)  # doctest: +ELLIPSIS
+    265252859812191058636308480000000...
     >>> factorial(-1)
     Traceback (most recent call last):
         ...
@@ -21,8 +21,8 @@ def factorial(n):
     Traceback (most recent call last):
         ...
     ValueError: n must be exact integer
-    >>> factorial(30.0)
-    265252859812191058636308480000000
+    >>> factorial(30.0)  # doctest: +ELLIPSIS
+    265252859812191058636308480000000...
 
     It must also not be ridiculously large:
     >>> factorial(1e100)