Update flake8 requirement from <7,>=5.0.4 to >=5.0.4,<8 (#1474)

requirements/testing.txt

@@ -5,7 +5,7 @@ pytest-asyncio<1  # for async
 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
+flake8>=5.0.4,<8
 #  Don't change this version without running CI builds;
 #  The latest version may not be available for older Python runtime
 black>=22.8.0; python_version=="3.6"