Hotfix of tholo/pytest-flake8#34

[changelog skip]

Author: dahlia

tox.ini

@@ -5,6 +5,10 @@ skipsdist = true
 [testenv]
 skip_install = true
 commands =
+    ; FIXME: the following command should be removed when
+    ; https://github.com/tholo/pytest-flake8/pull/35 is merged:
+    pip install git+git://github.com/jezdez/pytest-flake8.git@flake8-3.5.0
+
     pip install -f {distdir} -e .[tests]
     pytest -v