use flake8 < 3.6.0 to avoid new checks for old source

we won't fix the issues found by recently added style checks in
1.0-maint (they are already fixed in master).

tox.ini

@@ -17,5 +17,5 @@ passenv = *
 
 [testenv:flake8]
 changedir =
-deps = flake8
+deps = flake8<3.6.0
 commands = flake8