Use only one job for flake8

See https://gitlab.com/pycqa/flake8/issues/164

pytest.ini

@@ -1,3 +1,6 @@
 [pytest]
 addopts = --flake8 --isort --cov radicale/ radicale/tests
 norecursedirs = dist .cache .git build *.egg-info .eggs venv
+
+[flake8]
+jobs=1