Explicitly state no compatibility with flake8 >= 5.0.0 (#970)

The pycodestyle and pyflakes requirements added here are to deal with
incompatibilities introduced in newer versions of those flake8 plugins
with the older flake8 requirement of this package.

setup.py

@@ -34,12 +34,13 @@
         'PyYAML'],
     'extras_require': {
         'test': [
-            'flake8 >= 3.7',
+            'flake8 >= 3.7, < 5',
             'flake8-class-newline',
             'flake8_docstrings',
             'flake8-import-order',
             'pep8',
-            'pyflakes',
+            'pycodestyle < 2.9.0',
+            'pyflakes < 2.5.0',
             'pytest'],
     },
     'author': 'Dirk Thomas',