Pin flake8 version until flake8-quotes catches up. (#1227)

Author: n2ygk

tox.ini

@@ -90,7 +90,7 @@ basepython = python3.8
 skip_install = True
 commands = flake8 {toxinidir}
 deps =
-    flake8
+    flake8<6.0.0  # TODO remove this pinned version once https://github.com/zheller/flake8-quotes/pull/111 is merged.
     flake8-isort
     flake8-quotes
     flake8-black