Add constraint for pathspec.

This fixes installation of pathspec on Python 2.6.

changelogs/fragments/ansible-test-pathspec-constraint.yml

@@ -0,0 +1,2 @@
+bugfixes:
+    - ansible-test now limits ``pathspec`` to versions prior to 0.6.0 on Python 2.6 to avoid installation errors

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

@@ -25,6 +25,7 @@ requests-credssp >= 0.1.0 # message encryption support
 voluptuous >= 0.11.0 # Schema recursion via Self
 openshift >= 0.6.2, < 0.9.0 # merge_type support
 virtualenv < 16.0.0 ; python_version < '2.7' # virtualenv 16.0.0 and later require python 2.7 or later
+pathspec < 0.6.0 ; python_version < '2.7' # pathspec 0.6.0 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