Update pytest-cov requirement from <3,>=2 to >=2,<5 (#1476)

requirements/testing.txt

@@ -2,7 +2,7 @@
 aiohttp<4  # used for a WebSocket server mock
 pytest>=7.0.1,<8
 pytest-asyncio<1  # for async
-pytest-cov>=2,<3
+pytest-cov>=2,<5
 # while flake8 5.x have issues with Python 3.12, flake8 6.x requires Python >= 3.8.1,
 # so 5.x should be kept in order to stay compatible with Python 3.6/3.7
 flake8>=5.0.4,<7