Limit gssapi below v1.8.0 @ tests under Python 3.6

Ref: pythongssapi/python-gssapi#299

test/lib/ansible_test/_data/requirements/constraints.txt

@@ -15,3 +15,4 @@ mock >= 2.0.0 # needed for features backported from Python 3.6 unittest.mock (as
 pytest-mock >= 1.4.0 # needed for mock_use_standalone_module pytest option
 setuptools < 45 ; python_version == '2.7' # setuptools 45 and later require python 3.5 or later
 pyspnego >= 0.1.6 ; python_version >= '3.10' # bug in older releases breaks on Python 3.10
+gssapi < 1.8.0; python_version == '3.6'  # our test env only fails under RHEL+Python 3.6