bump flake8 version on test matrix

Author: nhymxu

.github/workflows/tests.yml

@@ -20,7 +20,7 @@ jobs:
       matrix:
         python-version: ['3.6', '3.7', '3.8', '3.9', '3.10', 'pypy3']
         isort: [4.3.21, 5.10.1]
-        flake8: [3.9.2, 4.0.1, 5.0.1]
+        flake8: [3.9.2, 4.0.1, 5.0.4]
 
     steps:
       - uses: actions/checkout@v2