[CI Fix] Remove ansible.module_utils.six imports (#998)

SUMMARY
This PR is essentially attempting Option B from issue #996 (Option A is implemented here); this code update accounts for the recent merge of sanity: warn on ansible.module_utils.six imports #85651.

Reviewed-by: Alina Buzachis
Reviewed-by: Yuriy Novostavskiy <yuriy@novostavskiy.kyiv.ua>

tests/unit/conftest.py

@@ -10,7 +10,6 @@ import ansible.module_utils.basic
 import pytest
 from ansible.module_utils._text import to_bytes
 from ansible.module_utils.common._collections_compat import MutableMapping
-from ansible.module_utils.six import string_types
 
 
 @pytest.fixture
@@ -20,7 +19,7 @@ def stdin(mocker, request):
     old_argv = sys.argv
     sys.argv = ["ansible_unittest"]
 
-    if isinstance(request.param, string_types):
+    if isinstance(request.param, str):
         args = request.param
     elif isinstance(request.param, MutableMapping):
         if "ANSIBLE_MODULE_ARGS" not in request.param: