Revert "Use only one job for flake8"

This reverts commit f4ebe3f5451c76dd7c33874e0bb4e011e9cd74f8.

pytest.ini

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