[stable-2.11] Restrict packaging to < 21.0 for Python < 3.6. (#75186)

(cherry picked from commit 67bc49e001)

Co-authored-by: Felix Fontein <felix@fontein.de>

changelogs/fragments/75186-ansible-test-packaging-constraint.yml

@@ -0,0 +1,2 @@
+bugfixes:
+- "ansible-test - restrict ``packaging`` to ``< 21.0`` for Python ``< 3.6`` (https://github.com/ansible/ansible/pull/75186)."

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

@@ -1,3 +1,4 @@
+packaging < 21.0 ; python_version < '3.6' # packaging 21.0 requires Python 3.6 or newer
 resolvelib >= 0.5.3, < 0.6.0  # keep in sync with `requirements.txt`
 coverage >= 4.5.1, < 5.0.0 ; python_version <  '3.7' # coverage 4.4 required for "disable_warnings" support but 4.5.1 needed for bug fixes, coverage 5.0+ incompatible
 coverage >= 4.5.2, < 5.0.0 ; python_version == '3.7' # coverage 4.5.2 fixes bugs in support for python 3.7, coverage 5.0+ incompatible