Update flake8 version (#6722)

This PR is useful for updating the flake8 checks we run, but is mostly
needed to update flake8 so that it can run on newer versions of python
which are included in newer ubuntu-latest versions from GitHub that we
update to in #6717

.pre-commit-config.yaml

@@ -65,7 +65,7 @@ repos:
         ]
 
 -   repo: https://github.com/pycqa/flake8
-    rev: 4.0.1
+    rev: 5.0.4
     hooks:
     -   id: flake8
         args: ['--config=.flake8']