Add pyyaml test constraint.

test/runner/requirements/constraints.txt

@@ -24,6 +24,7 @@ openshift >= 0.6.2 # merge_type support
 virtualenv < 16.0.0 ; python_version < '2.7' # virtualenv 16.0.0 and later require python 2.7 or later
 pyopenssl < 18.0.0 ; python_version < '2.7' # pyOpenSSL 18.0.0 and later require python 2.7 or later
 pyfmg == 0.6.1 # newer versions do not pass current unit tests
+pyyaml < 5.1 ; python_version < '2.7' # pyyaml 5.1 and later require python 2.7 or later
 pycparser < 2.19 ; python_version < '2.7' # pycparser 2.19 and later require python 2.7 or later
 mock >= 2.0.0 # needed for features backported from Python 3.6 unittest.mock (assert_called, assert_called_once...)
 pytest-mock >= 1.4.0 # needed for mock_use_standalone_module pytest option