Fix unit tests when running on Python 2.6

[Mathieu Bridon]

The unit tests were making use of facilities introduced in Python 2.7.

However, we need to build rpkg on EL 6, which has Python 2.6.

A solution could be to just not run the unit tests when building the EL 6
package, as the actual code of rpkg is properly compatible with Python 2.6.

However, to preserve confidence in our code on all paltforms where we ship it,
this patch series instead make the unit tests work with Python 2.6

 test/test_sources.py | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)