Style fixes in ha_tools.py.
Support Python 2.6+ with single source and "six" in high-availability tests.
Style fixes in test_motor_ha.
Support Python 2.6+ with single source and "six" in Motor and its unittests.
Support Python 2.6+ with single source and "six" in synchrotest.
More reliable test_copy_db_callback.
Replace "six" with a minimal internal module, "motor_py3_compat".