[2.7] bpo-24746: Fix doctest failures when running the testsuite with -R

[orsenthil]

[bpo-24746](https://bugs.python.org/issue24746): Fix doctest failures when running the testsuite with -R 

Cherry-pick: bpo-24746: c5dc60e to 2.7 branch

https://bugs.python.org/issue24746

$ ./python -m test test_doctest test_doctest
Run tests sequentially
0:00:00 load avg: 0.55 [1/2] test_doctest
0:00:00 load avg: 0.55 [2/2] test_doctest

== Tests result: SUCCESS ==

All 2 tests OK.

Total duration: 327 ms
Tests result: SUCCESS

[vstinner]

If "./python -m test test_doctest test_doctest" pass, you can merge the backport ;-)

[bedevere-bot]

@orsenthil: Please replace # with GH- in the commit message next time. Thanks!