Use different version of flake8 on python 3.7

Author: wch

setup.cfg

@@ -47,7 +47,7 @@ test =
     pytest>=6.2.4
 dev =
     black>=23.1.0
-    flake8==5.0.4;python_version<="3.7"
+    flake8==3.9.2;python_version<="3.7"
     flake8>=6.0.0;python_version>"3.7"
     isort>=5.11.2
     pyright>=1.1.284