python3Packages.markdown: downgrade pytest

test/test_markdown2.py - Failed: 'yield' keyword is allowed in fixtures, but not in tests (test_cases)

pkgs/development/python-modules/markdown2/default.nix

···
   fetchFromGitHub,
   latex2mathml,
   pygments,
-  pytestCheckHook,
+  pytest7CheckHook,
   pythonOlder,
   setuptools,
   wavedrom,
···
 
   pythonImportsCheck = [ "markdown2" ];
 
-  nativeCheckInputs = [ pytestCheckHook ];
+  nativeCheckInputs = [ pytest7CheckHook ];
 
   optional-dependencies = {
     code_syntax_highlighting = [ pygments ];