Update test constraints for Python 2.6. (#52055)

test/runner/requirements/constraints.txt

@@ -5,7 +5,7 @@ pywinrm >= 0.3.0 # message encryption support
 sphinx < 1.6 ; python_version < '2.7' # sphinx 1.6 and later require python 2.7 or later
 sphinx < 1.8 ; python_version >= '2.7' # sphinx 1.8 and later are currently incompatible with rstcheck 3.3
 wheel < 0.30.0 ; python_version < '2.7' # wheel 0.30.0 and later require python 2.7 or later
-yamllint != 1.8.0 ; python_version < '2.7' # yamllint 1.8.0 requires python 2.7+ while earlier/later versions do not
+yamllint != 1.8.0, < 1.14.0 ; python_version < '2.7' # yamllint 1.8.0 and 1.14.0+ require python 2.7+
 pycrypto >= 2.6 # Need features found in 2.6 and greater
 ncclient >= 0.5.2 # Need features added in 0.5.2 and greater
 idna < 2.6 # requests requires idna < 2.6, but cryptography will cause the latest version to be installed instead
@@ -23,6 +23,10 @@ pyfmg == 0.6.1 # newer versions do not pass current unit tests
 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
+xmltodict < 0.12.0 ; python_version < '2.7' # xmltodict 0.12.0 and later require python 2.7 or later
+lxml < 4.3.0 ; python_version < '2.7' # lxml 4.3.0 and later require python 2.7 or later
+pyvmomi < 6.0.0 ; python_version < '2.7' # pyvmomi 6.0.0 and later require python 2.7 or later
+pytest < 3.10.0 ; python_version >= '2.7' # pytest 3.10.0+ causes some tests to crash, pytest 4.2.0+ causes internal errors in pytest-forked
 
 # freeze pylint and its requirements for consistent test results
 astroid == 2.0.4

test/runner/requirements/units.txt

@@ -16,6 +16,7 @@ redis
 requests
 setuptools > 0.6 # pytest-xdist installed via requirements does not work with very old setuptools (sanity_ok)
 unittest2 ; python_version < '2.7'
+importlib ; python_version < '2.7'
 netaddr
 ipaddress
 netapp-lib