Avoid newer flake8

The new 6.0.0 contains a number of backwards incompatible changes
for which plugins need to be updated and configuration needs to be
updated.

Sadly the maintainer no longer accepts contributions or discussion
See PyCQA/flake8#1760

tox.ini

@@ -21,7 +21,7 @@ setenv=
 
 [testenv:flake8]
 skip_install = true
-deps = flake8
+deps = flake8<6.0
        flake8-author
        flake8-blind-except
        py{35,36,37,38,39,310}: flake8-bugbear