The subunit.test_results module is no longer imported by the top-level
subunit module and we have to explicitly import it.
This also fixes the broken doc build caused by the history.rst file.
The change log is now maintained differently so we don't need it
any more.
Closes-Bug: #2045766
Change-Id: Iae1ae0c55b8e1660a2bb41379a1f8f8506679cca