Add a note about flake8.

tox.ini

@@ -59,6 +59,7 @@ deps =
     # Twisted[http2] is required to import some files
     Twisted[http2]>=17.9.0
     pytest-flake8==1.1.1
+    # newer ones don't work: https://github.com/tholo/pytest-flake8/issues/87
     flake8==4.0.1
 commands =
     pytest --flake8 {posargs:docs scrapy tests}