Move isort configuration to setup.cfg

.isort.cfg

@@ -1,3 +0,0 @@
-[settings]
-known_standard_library=posixpath
-known_third_party=gunicorn,passlib,pkg_resources,pytest,pytest_cov,vobject

setup.cfg

@@ -7,3 +7,7 @@ python-tag = py3
 [tool:pytest]
 addopts = --flake8 --isort --cov -r s
 norecursedirs = dist .cache .git build Radicale.egg-info .eggs venv
+
+[isort]
+known_standard_library=posixpath
+known_third_party=gunicorn,passlib,pkg_resources,pytest,pytest_cov,vobject