Update dependency pytest-cov to v6

requirements_test.txt

@@ -8,6 +8,6 @@ mypy==1.11.2
 pymemcache==4.0.0
 pytest==8.3.3
 pytest-asyncio==0.24.0
-pytest-cov==4.1.0
+pytest-cov==6.0.0
 ruff==0.6.2
 safety==2.3.5